Ownership Submission
FORM 3
 UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
  Tseng Saria
 2. Date of Event Requiring Statement (Month/Day/Year)
02/09/2010		 3. Issuer Name and Ticker or Trading Symbol
MONOLITHIC POWER SYSTEMS INC [MPWR]
(Last)
(First)
(Middle)
6409 GUADALUPE MINES ROAD		 4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
VP and General Counsel
 5. If Amendment, Date Original Filed(Month/Day/Year)
(Street)

SAN JOSE, CA 95120		 6. Individual or Joint/Group Filing(Check Applicable Line)
_X_ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Beneficially Owned
1.Title of Security
(Instr. 4)		 2. Amount of Securities Beneficially Owned
(Instr. 4)		 3. Ownership Form: Direct (D) or Indirect (I)
(Instr. 5)		 4. Nature of Indirect Beneficial Ownership
(Instr. 5)
Common Stock 15,138 (1)
D
 

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly. SEC 1473 (7-02)
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Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 4)		 2. Date Exercisable and Expiration Date
(Month/Day/Year)		 3. Title and Amount of Securities Underlying Derivative Security
(Instr. 4)		 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 5)		 6. Nature of Indirect Beneficial Ownership
(Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Incentive Stock Option (right to buy) 10/15/2005(2) 10/15/2014 Common Stock 6,747 $ 8 D  
Non Qualified Stock Option (right to buy) 10/15/2005(2) 10/15/2014 Common Stock 49,001 $ 8 D  
Non Qualified Stock Option (right to buy) 07/27/2009(3) 07/27/2014 Common Stock 37,000 $ 16 D  
Non Qualified Stock Option (right to buy) 01/31/2010(4) 01/31/2015 Common Stock 10,000 $ 15.64 D  
Non Qualified Stock Option (right to buy) 10/22/2010(5) 10/22/2015 Common Stock 20,000 $ 13.01 D  

Reporting Owners

Reporting Owner Name / Address Relationships
Director 10% Owner Officer Other
Tseng Saria
6409 GUADALUPE MINES ROAD
SAN JOSE, CA 95120		       VP and General Counsel  

Signatures

/s/ Saria Tseng 02/10/2010
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 5(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Includes 3,000 restricted stock units. The restricted stock units vest in three equal installments on July 29, 2010, 2011 and 2012.
(2) A total of 150,000 shares were granted on 11/17/04. 50,000 shares vested on 10/15/05, and 1/24th of the shares vested monthly thereafter.
(3) A total of 37,000 shares were granted on 7/27/07. Fifty percent (50%) of the shares vested twenty-four (24) months after the grant date, and 1/48th of the shares vest monthly thereafter.
(4) A total of 10,000 shares were granted on 1/31/08. Fifty percent (50%) of the shares vested twenty-four (24) months after the grant date, and 1/48th of the shares vest monthly thereafter.
(5) A total of 20,000 shares were granted on 10/22/08. Fifty percent (50%) of the shares vest twenty-four (24) months after the grant date, and 1/48th of the shares vest monthly thereafter.

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