ITO films with thicknesses (134+8) nm, grown on glass substrates by sputtering method, were post-annealed at the temperatures of 100, 200, 300 and 400°C for 1 h, respectively. The as-deposited ITO film was amorphous, but crystallized with annealing at elevated temperatures, as demonstrated by X-ray diffraction. The transmittance spectra of all samples were obtained and subsequently simulated by means of spectroscopic ellipsometry. The optical constants n and k of the films were extracted. With the annealing temperature increasing, the optical constants n and k of the films firstly decreased then increased in the whole investigated wavelength range. The optical band gaps of all films were evaluated and they varied between 3.74 and 3.93 eV. ITO films with thicknesses (134 + 8) nm, grown on glass substrates by sputtering method, were post-annealed at the temperatures of 100, 200, 300 and 400 ° C for 1 h, respectively. The as-deposited ITO film was amorphous , but crystallized with annealing at elevated temperatures, as demonstrated by X-ray diffraction. The transmittance spectra of all samples were obtained and subsequently simulated by means of spectroscopic ellipsometry. The optical constants n and k of the films were extracted. With the annealing temperature increasing, the optical constants n and k of the films ground decreased then increased in the whole investigated wavelength range. The optical band gaps of all films were evaluated as and they varied between 3.74 and 3.93 eV.