Indium tin oxide (ITO) film with thickness of 140 nm was grown on Si substrate by sputtering method. X-ray diffraction (XRD) analysis shows that the film has the polycrystal structrue. The film was studied with reflecting spectroscopic ellipsometry in the energy range 1.5～4.5 eV. Parameterized analyses, based on Lorenz oscillators combined with Drude model, Graded model associated with Bruggeman effective-medium approximation model, were used to determine the optical constants and the thickness of the thin film. The results show that the indices of refraction of ITO film are in the range of 1.8～2.6 and the extinction coefficients are close to zero in the visible range and increase sharply near the wavelength of 350 nm. The optical band gap of the ITO film was estimated. A set of applicable and valuable data of dielectric coefficients and optical constants of the film were listed.