Using e-beam evaporation, the ellipsometric parameters of thick transparent films are studied with the modified analysis method for the SiO2 film samples deposited onto the Si substrate. The ellipsometric parameters are measured at the incidence angles changing from 50? to 70? and in the 3–4.5 eV photon energy range. The error in the conventional method can be significantly reduced by the modified ellipsometric method considering the spatial effect to show good agreement between the theoretical and experimental results. The new method presented in this letter can be applied to other optical measurement of the periodic or non-periodic film structures.