Abstract: The effects of thermally annealing on deep-levels of low temperature MBE GaAs have been investigated by Optical Transient Current Spectrum （OTCS）. There were three main deep level peaks LT₁, LT₂, LT₃ in the as-grown and annealed LT GaAs. The relative intensity of those peaks will change hugely after annealing. Gather and precipitation of arsenic by thermally annealing will decrease arsenic anti-site defects As_Ga and arsenic interstitial atom concentration that relative to LT₁. In opposite, the LT₃ that relative to vacancy V_Ga will increase.