As directly acquired by ellipsometer, the ellipsometric angles obtain their uncertainties from the instrument. As a metrological tool, one is interest in the derived parameters from the ellipsometric angles, therefore acquiring uncertainties from both instrument and material. As the relative variation of parameters are of interest, ellipsometers are very sensitive and precise. However, for extracting absolute values, calibration by independent method is necessary to elliminate the uncertaintie from the instrument and material. A framework for evaluating the uncertainties from the instrument related parameters, such as the wavelength, bandwidth, the angle of incidence, is given in this work. The framework could facilitate the use of ellipsometer for general measurement of various thin films other than limited type of films like SiO2 or SiNx on Si. For evaluating material related parameters, a typical application in the characterization of surface of silicon sphere is investigated by carefully investigating into the optical constants of sublayers.