Rapid thermal annealing (RTA) using halogen lamps for Si‐implanted (250‐keV, 2×1013cm−2dose) semi‐insulating liquid‐encapsulated Czochralski GaAs has been investigated by deep level transient spectroscopy. RTA has been performed at 700, 800 and 900 °C for 6 s with SiO2encapsulation. For comparison, conventional furnace annealing (FA) has also been performed at 800 °C for 20 min. In the RTA samples at 700 °C, three electron traps NI1 (Ec−0.53 eV), NI4 (Ec−0.63 eV), and EL2 (Ec−0.78 eV) are observed on the Cr‐doped substrate, while the traps NI1, EL2, and NI3 (Ec−0.47 eV) are observed on the undoped substrate. In RTA samples above 800 °C, the electron trap NI2 (Ec−0.55 eV) or EL2 is dominant and traps NI1, NI3, and NI4 are not detected. It is possible that three traps NI1, NI3, and NI4 are residual defects produced by Si implantation. The low concentration of the trap EL2 in FA samples is due to the out‐diffusion of it. This phenomenon can be related with the change of the stoichiometry in the implanted layer by the formation of the transition region at the SiO2/GaAs interface during FA. The notable spatial variation of the trap NI2 is observed across the wafer. On the other hand, the peculiar spatial distribution of the trap EL2 is not observed across the RTA sample. Furthermore, the trap EL2 concentration does not depend on RTA temperatures. It is possible that the spatial distribution of the trap EL2 across the RTA sample is changed by Si implantation and RTA.