<?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>0001229294</rptOwnerCik>
            <rptOwnerName>VENEGONI JOHN V</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/GM Surfactants</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>2203.3579</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6983.5768</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>522.4712</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>596.5314</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>596.5314</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">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>John V. Venegoni</signatureName>
        <signatureDate>2007-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-01-23
0000094049
STEPAN CO
SCL
0001229294
VENEGONI JOHN V
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
0
1
0
0
VP/GM Surfactants
Common Stock
2007-01-23
4
J
0
2203.3579
A
6983.5768
D
5-1/2% Convertible Preferred Stock
2007-01-23
4
J
0
522.4712
A
1988-08-08
1988-08-08
Common Stock
596.5314
596.5314
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.
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.
John V. Venegoni
2007-01-25