<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-08-01</periodOfReport>

    <issuer>
        <issuerCik>0000094049</issuerCik>
        <issuerName>STEPAN CO</issuerName>
        <issuerTradingSymbol>SCL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001351355</rptOwnerCik>
            <rptOwnerName>Servatius Gregory</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>22 W. FRONTAGE ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NORTHFIELD</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60093</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice Pres., Human Resources</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2433.4917</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8797.6677</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-08-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>933925</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>933925</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Member of Plan Committee of Stepan Company</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>518359</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>415566</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Member of Plan Committee of Stepan Company</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>5-1/2% Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>577.0414</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>1988-08-08</value>
            </exerciseDate>
            <expirationDate>
                <value>1988-08-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>658.837</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>658.837</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Number of common stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOP II) established by Stepan Company.  A total of 407,920 unallocated common stock shares was transferred to the ESOP II from the Stepan Company Profit Sharing Plan on December 27, 2006.</footnote>
        <footnote id="F2">Elected by the Stepan Company Board of Directors on August 1, 2006, to be a member of Plan Committee of Stepan Company, which has oversight responsibility over investments of funds of employee benefit programs.</footnote>
        <footnote id="F3">Member of Plan Committee of Stepan Company, which has oversight responsibility over investments of funds of employee benefit programs.  Amount represents number of Stepan Company stock shares transferred from the Stepan Company Profit Sharing Plan to the Employee Stock Ownership Plan II (ESOP II) on December 27, 2006.</footnote>
        <footnote id="F4">Number of preferred stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOP II ) established by Stepan Company.  A total of 96,728 unallocated preferred stock shares was transferred to the ESOP II from the Stepan Company Profit Sharing Plan on December 27, 2006.  The preferred stock conversion rate is 1.14175 shares of common stock for each share of preferred stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Gregory Servatius</signatureName>
        <signatureDate>2007-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-08-01
0000094049
STEPAN CO
SCL
0001351355
Servatius Gregory
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
0
1
0
0
Vice Pres., Human Resources
Common Stock
2007-01-23
4
J
0
2433.4917
A
8797.6677
D
Common Stock
2006-08-01
4
J
0
933925
A
933925
I
Member of Plan Committee of Stepan Company
Common Stock
2007-01-23
4
J
0
518359
D
415566
I
Member of Plan Committee of Stepan Company
5-1/2% Convertible Preferred Stock
2007-01-23
4
J
0
577.0414
A
1988-08-08
1988-08-08
Common Stock
658.837
658.837
D
Number of common stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOP II) established by Stepan Company.  A total of 407,920 unallocated common stock shares was transferred to the ESOP II from the Stepan Company Profit Sharing Plan on December 27, 2006.
Elected by the Stepan Company Board of Directors on August 1, 2006, to be a member of Plan Committee of Stepan Company, which has oversight responsibility over investments of funds of employee benefit programs.
Member of Plan Committee of Stepan Company, which has oversight responsibility over investments of funds of employee benefit programs.  Amount represents number of Stepan Company stock shares transferred from the Stepan Company Profit Sharing Plan to the Employee Stock Ownership Plan II (ESOP II) on December 27, 2006.
Number of preferred stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOP II ) established by Stepan Company.  A total of 96,728 unallocated preferred stock shares was transferred to the ESOP II from the Stepan Company Profit Sharing Plan on December 27, 2006.  The preferred stock conversion rate is 1.14175 shares of common stock for each share of preferred stock.
Gregory Servatius
2007-01-25