Ownership Submission
FORM 4
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN 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. Issuer Name and Ticker or Trading Symbol
MONOLITHIC POWER SYSTEMS INC [MPWR]
5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
VP & General Counsel
(Last)
(First)
(Middle)
79 GREAT OAKS BLVD
3. Date of Earliest Transaction (Month/Day/Year)
10/25/2018
(Street)

SAN JOSE, CA 95119
4. If Amendment, Date Original Filed(Month/Day/Year)
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 Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)
2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code
(Instr. 8)
4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)
5. Amount of Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 3 and 4)
6. Ownership Form: Direct (D) or Indirect (I)
(Instr. 4)
7. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V Amount (A) or (D) Price
Common Stock 10/25/2018(1)   M(1)   12,885 (1) A $ 0 297,295 D  
Common Stock 10/25/2018(2)   M(2)   15,075 (2) A $ 0 312,370 D  

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 3)
2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code
(Instr. 8)
5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4, and 5)
6. Date Exercisable and Expiration Date
(Month/Day/Year)
7. Title and Amount of Underlying Securities
(Instr. 3 and 4)
8. Price of Derivative Security
(Instr. 5)
9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 4)
10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 4)
11. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Performance Shares (1) 10/25/2018   M(1)     12,885   (1)   (1) Common Stock 12,885 $ 0 43,289 D  
Performance Shares (2) 10/25/2018   M(2)     15,075   (2)   (2) Common Stock 15,075 $ 0 28,214 D  
Restricted Stock Units (3) (3) 10/25/2018   (3) A   48,000     (3)   (3) Common Stock 48,000 $ 0 48,000 D  

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
Tseng Saria
79 GREAT OAKS BLVD
SAN JOSE, CA 95119
      VP & General Counsel  

Signatures

 /s/ Saria Tseng   10/29/2018
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(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) On December 31, 2015, the reporting person was granted a target number of 75,678 performance units in four tranches subject to satisfaction of share price targets over the next four years. The vesting of performance units in the second, third and fourth tranches is also subject to the satisfaction of certain operational goals, as determined by the Compensation Committee. All performance units are subject to time based vesting through January 1, 2020 or an earlier change in control. Once vested, one-third of the resulting shares have no sales restrictions and are sellable on January 1, 2020, one-third of the shares have sales restrictions until January 1, 2021 and one-third of the shares have sales restrictions until January 1, 2022. On October 25, 2018, the Compensation Committee determined one of the operational goals for the second tranche of performance units had been satisfied, which resulted in 12,885 units being credited, subject to time based vesting above.
(2) On December 31, 2015, the reporting person was granted a target number of 75,678 shares of performance units in four tranches subject to satisfaction of share price targets over the next four years. The vesting of performance units in the second, third and fourth tranches is also subject to the satisfaction of certain operational goals, as determined by the Compensation Committee. All performance units are subject to time based vesting and require service through January 1, 2020 or an earlier change in control.Once vested, one-third of the resulting shares have no sales restrictions and are sellable on January 1, 2020, one-third of the shares have sales restrictions until January 1, 2021 and one-third of the shares have sales restrictions until January 1, 2022.On October 25, 2018,the Compensation Committee determined one of the operational goals for the third tranche of performance units had been satisfied, which resulted in 15,075 units being credited, subject to time based vesting.
(3) On October 25, 2018, the reporting person was granted a target number of 9,600 performance units and can earn up to a maximum of 48,000 performance units subject to satisfaction of share price targets ranging from $140 to $172 over the performance periods from October 25, 2018 to December 31, 2023, as determined by the Compensation Committee of the Company. All performance units are subject to time based vesting and require service through January 1, 2024 or an earlier change in control. Once vested, performance units convert into shares of common stock on a 1-for-1 basis and one-third of the resulting shares have no sales restrictions and are sellable on January 1, 2024, one-third of the shares have sales restrictions until January 1, 2025 and one-third of the shares have sales restrictions until January 1, 2026.

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