The ellipsometric method has been developed to investigate the radiation damage and annealing effect in As ion implanted silicon. For 1×1016 cm-2, 150 keV As ion implantation, the refractive index-depth profile exhibits a plateau form, indicating the existence of an amorphous layer. The annealing temperature used to remove amorphous layer must be about 700℃ or higher. The experimental results show that the ellipso-meter is one of the useful tools for radiation damage determinations.