An apparatus for measuring the ellipsometric spectra has been set up in our laboratory. From the experimental values of Imax,Imin and θmin, it is easy to determine the ellipsometric parameters ψ, △ as functions of wavelengths. The ellipsometric spectra of SiO2 films of several thicknesses on silicon have been determined. When comparing the experimental data with the theoretical curves calculated by computer, we find that they are generally in agreement. The ellipsometric spectrum method for measuring the film thickness has also been compared with the extinction method.