In this paper,the dependence of the optical constants on the photonenergy for two samples of intrinsic and doped Si Al xGa 0.51-x In 0.49 P were studied by using ellipsometric spectroscopy.The dielectric function spectra of two samples were obtained in the region of visible light,and the third derivative spectra of the dielectric function were evaluated.The energy gaps for two samples were analyzed by the third derivative theory.The energy gaps were gained and discussed.