The results of investigation of optical transmission spectra and X-ray diffraction of thin In2O3 films deposited by dc-magnetron sputtering on Al2O3 (012) substrates are presented. The diffraction patterns exhibit the presents of the (222) reflex of cubic In2O3. Its position shifts from 30.3 to 30.6°, with a decrease in the film thickness. There was also a decrease in the half-width of this reflex by a decrease in the deposition time, which may indicate an increase in the crystallite size of the film material. According to the optical transmission measurements, the presence of a transition layer with the band gap of 1.39 eV and about of 40 nm thickness was established at the interface between the film and substrate. The properties of this layer are independent of the deposition time.