Optical constants of Al xGa 0.51-x In 0.49 P and that doped by Si prepared by MOCVD on the GaAs substrates were measured by using the ellipsometric spectroscopy in the visible light region at room temperature.Dependence of the absorption coefficients and dielectric functions for two samples on the photon energy were obtained.The Al composition was calculated by using EMA and linear interpolation,and the results was compared with that obtained by the energy spectrum method.Three results were consistent with each other.