<?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>0001229293</rptOwnerCik>
            <rptOwnerName>PACHOLEC FRANK</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 President of R &amp; D</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>51.19</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11901.19</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>5 1/2% Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>12.1386</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </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>13.86</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>13.86</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 (ESOPII) established by Stepan Company.  A total of 407,920 unallocated common stock shares was transferred to the ESOP II from Stepan Company Profit Sharing Plan on December 27, 2006.</footnote>
        <footnote id="F2">Number of preferred stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOPII) established by Stepan Company.  A total of 96,728 unallocated preferred stock shares was transferred to the ESOP II from 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>Frank Pacholec</signatureName>
        <signatureDate>2007-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-01-23
0000094049
STEPAN CO
SCL
0001229293
PACHOLEC FRANK
22 W FRONTAGE ROAD
NORTHFIELD
IL
60093
0
1
0
0
Vice President of R & D
Common Stock
2007-01-23
4
J
0
51.19
A
11901.19
D
5 1/2% Convertible Preferred Stock
2007-01-23
4
J
0
12.1386
A
1988-08-08
1988-08-08
Common Stock
13.86
13.86
D
Number of common stock shares allocated to reporting person's account on January 23, 2007, under the Employee Stock Ownership Plan II (ESOPII) established by Stepan Company.  A total of 407,920 unallocated common stock shares was transferred to the ESOP II from Stepan Company Profit Sharing Plan 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 (ESOPII) established by Stepan Company.  A total of 96,728 unallocated preferred stock shares was transferred to the ESOP II from 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.
Frank Pacholec
2007-01-25