Abstract: The technique of photo induced transient spectroscopy （PITS） has been used to observe thermal behaviors of deep-level defects in annealed SI-GaAs. With isochronic rapid thermal annealing （RTA） at temperatures ranging from 500℃ to 800℃,PITS signals of EL2 （Ec-0.82eV） and EL12 （Ec-0.79eV） strengthen first and then get saturated,and those of EL6 （Ec-0.38eV）, EL8（Ec-0.27eV） and EL9 （Ec-0.24eV） reach maxima at 600℃ or so and then weaken. These indicate that they may respectively be close in structures and belong to EL2 and EL6 group. Besides,after RTA （lower than 900℃）,the quantity of defects of the EL2 group increases,while that of the EL6 group decreases,compared with the case of as-grown samples. By furnace annealing at 950℃ for 5 hours, however, defects of the EL2 group break up,while defects of the EL6 group engender. From the experiment it can be seen that the EL2 group （EL2, EL12） and the EL6 group （EL6, EL8 and EL9） are related in microscopic structure. And it might be resonable assuming that EL12 is As_Gα V^As V_Gα and EL6 is VAs V_Gα.