Aluminum oxide thin films were prepared on polished silicon wafers by radio frequency sputtering. The optical constants and the thicknesses of the deposited layers were determined by spectroscopic ellipsometry in the wavelength range from 191 nm to 1690 nm. The dielectric function of the aluminum oxide thin films was modelled by the combination of the Cauchy dispersion relation and the Urbach absorption tail. The optical properties of the sputtered films were compared with those prepared by atomic layer deposition, electron beam evaporation, and pulsed magnetron sputtering in other laboratories.