Indium tin oxide (ITO) and Ag (1.2 +/- 0.1 at. %)-ITO films with the thickness of 130 nm were deposited on glass substrates at room temperature by dc magnetron sputtering and postannealed at the temperature range of 200-400 degrees C. By calculating the x-ray diffraction data, the lattice constants of all samples were obtained and the results show that the annealing led to the smaller lattice constants and the Ag doping resulted in the further lattice distortion. The refractive index n and extinction coefficient k of all samples were extracted from the transmittance spectra by means of the spectroscopic ellipsometry optimization method. Ag-ITO film annealed at 400 degrees C has the high transmittance of 80%-90% in the visible wavelength range. Ag doping dramatically increased the extinction coefficient k of ITO films in UV wavelength range but almost without change in it in visible band. Meanwhile, by contrast with ITO films, Ag-ITO films show much higher n values than that of ITO films. Finally, the optical band gaps of all samples were determined, and it has been found that there is almost no difference of band gaps between ITO and Ag-ITO films. The reasons of the influence of annealing and Ag doping on structural and optical properties of ITO thin films are discussed. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3264478]