This paper analyzes the degradation in GaAs pseudomorphic high electron mobitity transistors (PHEMT's) by measuring the electric characteristics in GaAs PHEMT's before and after stress. The relation between impact ionization rate and maximu m channel electric field is gained. An analytical expression of impact ionizatio n rate versus maximum channel electric field is deduced by fitting the experimen tal results. The electric characteristic and reliability in GaAs PHEMT's can be improved and evaluated using the analytical expression. The impact ionization in channel should be decreased in order to improve the breakdown voltage in GaAs P HEMT's.