<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

<documentType>4</documentType><periodOfReport>2003-05-02</periodOfReport>
<issuer><issuerCik>0001109357</issuerCik>
        <issuerName>EXELON CORP</issuerName>
<issuerTradingSymbol>EXC</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId><rptOwnerCik>0001184021</rptOwnerCik>
            <rptOwnerName>MOLER ELIZABETH A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
<reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Executive Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Deferred Comp. - Phantom Shares</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F1"/></conversionOrExercisePrice><transactionDate><value>2003-05-02</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>10</value></transactionShares><transactionValue><value>53.87</value></transactionValue><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>10</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>601</value><footnoteId id="F1"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Phantom shares held in a multi-fund deferred compensation account to be settled for cash on a 1:1 basis upon the reporting person's termination of employment for any reason.  Shares are acquired through regular periodic payroll contributions and the reinvestment of dividends. Balance also includes 6 shares acquired on 03/10/2003 through the automatic dividend reinvestment feature of Exelon plans.</footnote></footnotes><ownerSignature><signatureName>Scott N. Peters, Attorney in Fact for Elizabeth A. Moler</signatureName><signatureDate>2003-05-06</signatureDate></ownerSignature></ownershipDocument>

