In order to develop the ellipsometry used for determination of ion implanted damage layer, we have measured the ellipsometric spectrum of ion implanted silicon in a wavelength range of 4000-7000?. The samples were implanted by 150 keV, l015/cm2 or 1016/cm2 As ions. In this case the amorphous layers were formed at the surface of Si samples, so that a monolayer model could be used to calculate the relations of (n, k)-λ from the data of (ψ, Δ)-λ. The peak of n-λ curve of As ion implanted silicon is situated at ～4800?, and the peak value is about 4.9. The experimental results of ion implanted silicon have been compared with those of sputtering amorphous silicon, evaporated amorphous silicon and single crystalline silicon.