The characterization and thermal annealing behavior of deep-level defects in 1 MeV neutron irradiated VPE n-GaAs layers have been studied. The results indicate that the majority of induced defects are associated with displacement of two or more neighboring atoms. In addition, a new defect level E5(Ec-0.73 eV) emerges, as the H1 level anneals out, It is found that the changes of GaAs MESFET parameters are mainly due to the carrier removal.