The behaviors of Si ions implanted into (100) GaAs at liquid nitrogen temperature with energy of 48 keV at the doses of 1×10¹⁵—5×10¹⁵ ions / cm² has been investigated in this study. The Rutherford backscattering-channeling (RBS-C) combined with particle induced X-ray emission (PIXE) has been used to determine the sites of the Si atoms in the GaAs substrate. The four-point probe was used to measure the resistance of the GaAs before and after Si ions implantation. The experimental results show that Si atoms occupy not only on Ga site but also on As site. The sheet resistivity of GaAs reduced from 1×10⁹Ω/□ to 4.5×10⁶Ω /□ after Si ions implanted, and to 4.0 × 10⁴Ω/□ after annealing at 850°C in argon. These results are consistent with some other investigations, for instance, the results of G.Braunstein et al and R. S. Bhattacharya et al, although the implantation condition is not the same.