<?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>0001280011</rptOwnerCik>
            <rptOwnerName>WOOD ROBERT JOHN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>22 W. FRONTAGE RD.</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 &amp; GM Polymers</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>2602.4353</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4825.4353</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>617.1022</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>704.5764</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>704.5764</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>Robert John Wood</signatureName>
        <signatureDate>2007-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-01-23
0000094049
STEPAN CO
SCL
0001280011
WOOD ROBERT JOHN
22 W. FRONTAGE RD.
NORTHFIELD
IL
60093
0
1
0
0
VP & GM Polymers
Common Stock
2007-01-23
4
J
0
2602.4353
A
4825.4353
D
5 1/2% Convertible Preferred Stock
2007-01-23
4
J
0
617.1022
A
1988-08-08
1988-08-08
Common Stock
704.5764
704.5764
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.
Robert John Wood
2007-01-25