Abstract: The tliermal model of steady state temperature in microwave GaAs power FET Chips,hased on its geometry structure feature,is developed, its nulnerical solution is obtained with three dimension finite element rnethods,The factors which influence on chip surface channel temperature are analysed.including its power dissipation,its ambicnl lemperature,Ga-As material thermal conductivity,chio thickness,heat source distribution in the chip.FETs transcondtictance.