<?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>0001229309</rptOwnerCik>
            <rptOwnerName>CADIEUX ROBERT D</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>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Directors Deferred Compensation Plan</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>31.67</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-01-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>28592</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>31.67</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>28592</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>586</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On January 23, 2007, 28,592 shares were disposed of from the Directors Deferred Compensation Plan account and paid in cash to the Director pursuant to the provisions of the Directors Deferred Compensation Plan Amended and Restated as of January 1, 2005.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kathleen M. Owens, by Power of Attorney for Robert D. Cadieux</signatureName>
        <signatureDate>2007-01-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-01-23
0000094049
STEPAN CO
SCL
0001229309
CADIEUX ROBERT D
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
1
0
0
0
Directors Deferred Compensation Plan
31.67
2007-01-23
4
J
0
28592
31.67
D
1988-08-08
1988-08-08
Common Stock
28592
586
D
On January 23, 2007, 28,592 shares were disposed of from the Directors Deferred Compensation Plan account and paid in cash to the Director pursuant to the provisions of the Directors Deferred Compensation Plan Amended and Restated as of January 1, 2005.
Kathleen M. Owens, by Power of Attorney for Robert D. Cadieux
2007-01-24