Abstract: The coupled Maxwell's equations, heat transport equation and carrier equations are solved to simulate the change of temperature and scatering parameters of the GaAs MESFET which is under the irradiation of high power electromagnetic pulses. The transient temperature response curves are also calculated. The effects of electromagnetic pulse sequences with different magnitude and pulses with different rise time are studied and compared. The global modeling method and the heat transport equation are coupled for the first time. This method is based on physical modeling, taking into account the electromagnetic coupling, radiation effects and electron-wave interaction. The results show that the temperature between the gate and the source rises rapidly and the scattering parameters deteriorate dra