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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-01-23</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001229292</rptOwnerCik>
            <rptOwnerName>HURLBUTT JAMES E</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>VP, Finance</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>2478.233</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9000.82</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></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="F2"/>
                </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="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>587.6507</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F3"/>
                </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>670.9501</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>670.9501</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">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="F3">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>James E. Hurlbutt</signatureName>
        <signatureDate>2007-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-01-23
0000094049
STEPAN CO
SCL
0001229292
HURLBUTT JAMES E
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
0
1
0
0
VP, Finance
Common Stock
2007-01-23
4
J
0
2478.233
A
9000.82
D
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
587.6507
A
1988-08-08
1988-08-08
Common Stock
670.9501
670.9501
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.
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.
James E. Hurlbutt
2007-01-25