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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-02-14</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001229311</rptOwnerCik>
            <rptOwnerName>GROJEAN THOMAS F</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>26.25</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-02-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>571</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>26.25</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>571</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18105</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On February 14, 2006, 571 shares were added to the Directors Deferred Compensation Plan account as per the provisions in the First Amendment to the Stepan Company Directors Deferred Compensation Plan.  The 571 shares shall be paid to the director upon termination of service on the Board of Directors and shall be paid only in shares of Stepan Company Common Stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kathleen M. Owens, by Power of Attorney for Thomas F. Grojean</signatureName>
        <signatureDate>2006-02-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-02-14
0000094049
STEPAN CO
SCL
0001229311
GROJEAN THOMAS F
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
1
0
0
0
Directors Deferred Compensation Plan
26.25
2006-02-14
4
J
0
571
26.25
A
1988-08-08
1988-08-08
Common Stock
571
18105
D
On February 14, 2006, 571 shares were added to the Directors Deferred Compensation Plan account as per the provisions in the First Amendment to the Stepan Company Directors Deferred Compensation Plan.  The 571 shares shall be paid to the director upon termination of service on the Board of Directors and shall be paid only in shares of Stepan Company Common Stock.
Kathleen M. Owens, by Power of Attorney for Thomas F. Grojean
2006-02-16