-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 OEr6/WTgeU8OfJBOiPRGskQWmzQz1kOU3iLsnVWnL++x9JtF9wRXLc4QdHmzqFsq
 MHZ6rwL2+dGN81M7cp2fKw==

<SEC-DOCUMENT>0001280452-09-000123.txt : 20090924
<SEC-HEADER>0001280452-09-000123.hdr.sgml : 20090924
<ACCEPTANCE-DATETIME>20090924170432
ACCESSION NUMBER:		0001280452-09-000123
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090924
FILED AS OF DATE:		20090924
DATE AS OF CHANGE:		20090924

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Moyer James C
		CENTRAL INDEX KEY:			0001308656

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-51026
		FILM NUMBER:		091085503

	MAIL ADDRESS:	
		STREET 1:		6409 GUADALUPE MINES ROAD
		CITY:			SAN JOSE
		STATE:			CA
		ZIP:			95120

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MONOLITHIC POWER SYSTEMS INC
		CENTRAL INDEX KEY:			0001280452
		STANDARD INDUSTRIAL CLASSIFICATION:	SEMICONDUCTORS & RELATED DEVICES [3674]
		IRS NUMBER:				770466789
		STATE OF INCORPORATION:			CA
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		6409 GUADALUPE MINES ROAD
		CITY:			SAN JOSE
		STATE:			CA
		ZIP:			95120
		BUSINESS PHONE:		(408) 826-0600

	MAIL ADDRESS:	
		STREET 1:		6409 GUADALUPE MINES ROAD
		CITY:			SAN JOSE
		STATE:			CA
		ZIP:			95120
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgardoc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-09-24</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001280452</issuerCik>
        <issuerName>MONOLITHIC POWER SYSTEMS INC</issuerName>
        <issuerTradingSymbol>MPWR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001308656</rptOwnerCik>
            <rptOwnerName>Moyer James C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>6409 GUADALUPE MINES ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN JOSE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>95120</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief IC Design Engineer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-09-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>720</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>23.4381</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>494524</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by Moyer FamilyTrust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>890569</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">In accordance with the reporting person's 10b5-1 trading plan.</footnote>
        <footnote id="F2">The price is the weighted average sale price for the transactions reported on this line.  The prices for the transactions reported on this line range from $23.25 to $23.72.  The reporting person undertakes to provide, upon request by the staff of the Securities and Exchange Commission, the issuer, or a security holder of the issuer, full information regarding the number of shares sold at each separate price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>By: Adriana Chiocchi  For: James Moyer</signatureName>
        <signatureDate>2009-09-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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