We investigated the optical properties of AlN films with different thicknesses grown on sapphire by spectroscopic ellipsometry at different temperature. Based on a Tauc-Lorentz dispersion model, thickness and optical constants (the refractive index n, the extinction coefficient k) of AlN films were extracted by fitting the experimental data. Our results show that the refractive index of thicker AlN film possesses bigger values. Similar to the previous report, it was also found that the refractive index, the extinction coefficient and band gap of AlN films shift monotonously to lower energies (a redshift) with temperature increasing. Moreover, with rising temperature, varying the thicknesses of the films exhibits little influence on the shrinkage of bandgap but slight influence on the changes of the refractive index.