The defect behavior of semi-insulating（SI） GaAs samples has been measured by the positron lifetime spectrometer at different isochronal annealing temperatures before and after neutron irradiation. It is found that vacancy-interstitial pairs and vacancy-antisite defects begin to recombine at 70℃

and vacancies and voids will disappear before 500℃

but new defects may reproduce after 740℃. The positron binding energy of shallow traps（VAs）is estimated to be 0.031-0.032eV.