¨       

Preliminary Proxy Statement

¨       

Confidential, for Use of the Commission Only

(as permitted by Rule 14a-6(e)(2))

¨       

Definitive Proxy Statement

x       

Definitive Additional Materials

¨       

Soliciting Material Pursuant to Rule 14a-12

The Proxy Statement, Annual Report and other proxy materials are available at:

http://www.proxyvoting.com/mpwr

If you want to receive a paper or e-mail copy of these documents, you must request one. There is no charge to you for requesting a copy. Please make your request for a copy as instructed below on or before May 27, 2010 to facilitate timely delivery.

TO REQUEST PAPER COPIES OF PROXY MATERIALS:

(please reference your 11-digit control number when requesting materials)

By opting out to receive printed materials, your preference for future proxy mailings will be kept on our file.

Telephone:

1-888-313-0164

Email:

shrrelations@bnymellon.com

Internet:

http://www.proxyvoting.com/mpwr

TO VOTE YOUR SHARES SEE INSTRUCTIONS ON REVERSE SIDE

This is not a proxy card. You cannot use this notice to vote your shares.

YOU MUST REFERENCE YOUR 11-DIGIT CONTROL NUMBER WHEN

YOU REQUEST A PAPER COPY OF THE PROXY MATERIALS OR TO

VOTE YOUR PROXY ELECTRONICALLY.

the Company’s 2010 Proxy Statement (including all attachments thereto);

the Company’s Annual Report for the year ended December 31, 2009 (which is not deemed to be part of the official proxy soliciting materials); and

any amendments to the foregoing materials that are required to be furnished to stockholders.

HOW TO VOTE BY INTERNET

We encourage you to review the proxy materials online before voting.

Use the Internet to vote your shares. On the landing page of the above website in the box labeled

“To Vote Your Shares by Internet” click on “Vote Now” to access the electronic proxy card and vote your shares.

Have this notice in hand when you access the website.

You will need to reference the 11-digit control number located on the reverse side.

MONOLITHIC POWER SYSTEMS, INC.

Meeting Information

Meeting Type: Annual Meeting

For holders as of : April 19, 2010

Date: June 10, 2010        Time: 10:00 AM PDT

Location:    Monolithic Power Systems, Inc.

6409 Guadalupe Mines Road

San Jose, CA 95120

You are receiving this communication because you hold shares in the above named company.

This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side).

We encourage you to access and review all of the important information contained in the proxy materials before voting.

Return Address Line 1

Return Address Line 2

Return Address Line 3

51 MERCEDES WAY

EDGEWOOD NY 11717

Investor Address Line 1

Investor Address Line 2

Investor Address Line 3

Investor Address Line 4

Investor Address Line 5

John Sample

1234 ANYWHERE STREET

ANY CITY, ON A1A 1A1

Broadridge Internal Use Only

Job #

Envelope #

Sequence #

# of # Sequence #

Before You Vote  

  1. Annual Report         2. Notice & Proxy Statement

  How to View Online:

  Have the 12-Digit Control Number available (located on the following page) and visit: www.proxyvote.com.

  How to Request and Receive a PAPER or E-MAIL Copy:

  If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a   copy. Please choose one of the following methods to make your request:

                                 1) BY INTERNET:         www.proxyvote.com

                                 2) BY TELEPHONE:     1-800-579-1639

                                 3) BY E-MAIL*:             sendmaterial@proxyvote.com

  *      If requesting materials by e-mail, please send a blank e-mail with the 12-Digit Control Number (located on the following page) in the subject line.

  Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor.   Please make the request as instructed above on or before May 27, 2010 to facilitate timely delivery.

How To Vote  

Vote In Person: If you choose to vote these shares in person at the meeting, you must request a “legal proxy.” To do so, please follow the instructions at www.proxyvote.com or request a paper copy of the materials, which will contain the appropriate instructions. Many shareholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance.

Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the 12-Digit Control Number available and follow the instructions.

Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a voting instruction form.

Voting items

vote FOR the following:

The Board of Directors recommends you vote FOR the following proposal (s):

VOTE TO RATIFY THE APPOINTMENT OF DELOITTE & TOUCHE LLP AS THE INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM OF MONOLITHIC POWER SYSTEMS, INC FOR THE FISCAL YEAR ENDING DECEMBER 31, 2010.

NOTE: Such other business as may properly come before the meeting or any adjournment thereof.

Reserved for Broadridge Internal Control Information

Voting Instructions

THIS SPACE RESERVED FOR LANGUAGE PERTAINING TO

BANKS AND BROKERS

AS REQUIRED BY THE NEW YORK STOCK EXCHANGE

THIS SPACE RESERVED FOR SIGNATURES IF APPLICABLE