Abstract: The response of metal-oxide -semiconductor field effect transistor (MOSFET) under high power microwaves (HPMs) is numerically studied by a simulator based on semiconductor drift-diffusion model. Output property and internal response of MOSFET is numerically simulated. Bias voltage may be reduced to less than threshold by the HPM signal injected in to gate electrode and results in a waveform change of output current. Inside the device, electric field intensity in one end of the channel close to the drain electrode is much higher than at the other places, and heat source is concentrated in this zone. In positive semi-cycles peak, the peak value of temperature is in one end of channel close to the drain electrode. In negative semi-cycles, the distribution of temperature tends to be uniform in the middle of the channel. The simulation result can be applied in damage mechanism analysis and design of MOSFET hardening against HPMs.