The thickness and dielectric constants of thin films usually have certain correlation in the fitting procedure of spectroscopic ellipsometry (SE). The choice of different dispersion models may also influence the results and cause errors. As the fitting is influenced by the dispersion models adopted in the analysis, the uniqueness test has been introduced into SE fitting. The results of uniqueness test have been compared with different dispersion models, different film thicknesses, different wavelength ranges and different incident angles using titanium dioxide samples as an example. It is indicated that uniqueness test is efficient in evaluating the fitting for SE measurement. Uniqueness test can also provide quantitative comparison among different dispersion models and contribute to fitting precision.