-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 QYrKXmlCIVsa24rBxWdS020X9ROwQrvfHUWggc/KJqlUUY0MwpTnqhoGHeaqvfAL
 QUGlSVnQHo+nFLHd2IO2/Q==

<SEC-DOCUMENT>0001249908-06-000003.txt : 20060511
<SEC-HEADER>0001249908-06-000003.hdr.sgml : 20060511
<ACCEPTANCE-DATETIME>20060511182805
ACCESSION NUMBER:		0001249908-06-000003
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20060509
FILED AS OF DATE:		20060511
DATE AS OF CHANGE:		20060511

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			NORTHWEST PIPE CO
		CENTRAL INDEX KEY:			0001001385
		STANDARD INDUSTRIAL CLASSIFICATION:	STEEL PIPE & TUBES [3317]
		IRS NUMBER:				930557988
		STATE OF INCORPORATION:			OR
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		12005 N BURGARD
		STREET 2:		P O BOX 83149
		CITY:			PORTLAND
		STATE:			OR
		ZIP:			97203
		BUSINESS PHONE:		5032851400

	MAIL ADDRESS:	
		STREET 1:		12005 N BURGARD
		STREET 2:		P O BOX 83149
		CITY:			PORTLAND
		STATE:			OR
		ZIP:			97203

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ROMAN RICHARD A
		CENTRAL INDEX KEY:			0001234059

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-27140
		FILM NUMBER:		06831679

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		5032283226

	MAIL ADDRESS:	
		STREET 1:		1984 SW 16TH AVE
		CITY:			PORTLAND
		STATE:			OR
		ZIP:			97201
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-05-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001001385</issuerCik>
        <issuerName>NORTHWEST PIPE CO</issuerName>
        <issuerTradingSymbol>NWPX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001234059</rptOwnerCik>
            <rptOwnerName>ROMAN RICHARD A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>200 SW MARKET ST #1800</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PORTLAND</rptOwnerCity>
            <rptOwnerState>OR</rptOwnerState>
            <rptOwnerZipCode>97201</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>NON-QUALIFIED STOCK OPTION</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.31</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-05-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2006-11-09</value>
            </exerciseDate>
            <expirationDate>
                <value>2016-05-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>COMMON STOCK</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <ownerSignature>
        <signatureName>JOHN D. MURAKAMI FOR RICHARD ROMAN</signatureName>
        <signatureDate>2006-05-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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