EXHIBIT 5.1

OPINION OF JONES DAY

February 2, 2011

Monolithic Power Systems, Inc.

6409 Guadalupe Mines Road

San Jose, CA 95120

Re:           Registration Statement on Form S-8 Filed by Monolithic Power Systems, Inc.

Dear Ladies and Gentlemen:

We have acted as counsel for Monolithic Power Systems, Inc., a Delaware corporation (the “Company”), in connection with the registration under the Securities Act of 1933, as amended (the “Act”), of an aggregate of 2,454,411 shares (the “Shares”) of common stock, par value $0.001 per share, of the Company that may be issued or delivered and sold pursuant to the Company’s 2004 Equity Incentive Plan and the Company’s 2004 Employee Stock Purchase Plan (collectively, the “Plans”).  In connection with the opinion expressed herein, we have examined such documents, records and matters of law as we have deemed relevant or necessary for purposes of this opinion.  Based on the foregoing, and subject to the further limitations, qualifications and assumptions set forth herein, we are of the opinion that the Shares that may be issued or delivered and sold pursuant to the Plans have been authorized by all necessary corporate action and will be, when issued or delivered and sold in accordance with such Plans and the authorized forms of stock option, restricted stock or other applicable agreements thereunder, validly issued, fully paid and nonassessable, provided that the consideration for the Shares is at least equal to the stated par value thereof.

The opinion expressed herein is limited to the General Corporation Law of the State of Delaware, including the applicable provisions of the Delaware Constitution and the reported judicial decisions interpreting such law, as currently in effect, and we express no opinion with respect to any other law of the State of Delaware or the laws of any other jurisdiction.  In addition, we have assumed that the resolutions authorizing the Company to issue or deliver and sell the Shares pursuant to the Plans and the applicable award agreements will be in full force and effect at all times at which such Shares are issued or delivered or sold by the Company, and the Company will take no action inconsistent with such resolutions.

In rendering the opinion above, we have assumed that each award under the 2004 Equity Incentive Plan will be approved by the Board of Directors of the Company or an authorized committee of the Board of Directors.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement on Form S-8 filed by the Company to effect registration of the Shares to be issued or delivered and sold pursuant to the Plans under the Act.  In giving such consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Securities and Exchange Commission promulgated thereunder.

Very truly yours,

/s/ JONES DAY

Jones Day