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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-09-29</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001139402</rptOwnerCik>
            <rptOwnerName>POTTER ROBERT G</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>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Directors Deferred Compensation Plan</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>29.23</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-09-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>359</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.23</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</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>359</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11352</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Directors Deferred Compensation Plan</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>29.23</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-10-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>352</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.23</value>
                </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>352</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11704</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On September 29, 2006, 359 shares were disposed of from the Directors Deferred Compensation Plan account and paid in cash to the Director pursuant to the provisions of the Director's Agreement with Stepan Company.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kathleen M. Owens, by Power of Attorney for Robert G. Potter</signatureName>
        <signatureDate>2006-10-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-09-29
0000094049
STEPAN CO
SCL
0001139402
POTTER ROBERT G
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
1
0
1
0
Directors Deferred Compensation Plan
29.23
2006-09-29
4
J
0
359
29.23
D
1988-08-08
1988-08-08
Common Stock
359
11352
D
Directors Deferred Compensation Plan
29.23
2006-10-02
4
J
0
352
29.23
A
1988-08-08
1988-08-08
Common Stock
352
11704
D
On September 29, 2006, 359 shares were disposed of from the Directors Deferred Compensation Plan account and paid in cash to the Director pursuant to the provisions of the Director's Agreement with Stepan Company.
Kathleen M. Owens, by Power of Attorney for Robert G. Potter
2006-10-02