ITO thin films were deposited on the glass substrates by microwave-enhanced d.c. reactive magnetron sputtering technique at different oxygen partial pressures (3.8-11.7 X 10(-4) mbar). The structural, electrical and optical properties of the deposited films have been studied. The films prepared at low oxygen pressure show an amorphous compact structure and the films prepared at high oxygen pressure show a polycrystalline structure with a preferred orientation along the (222) direction. The electrical resistivity of the films increases as the oxygen partial pressure is increased. The optical properties were studied by measuring the transmittance and the ellipsometric spectra. The optical constants of the films in the telecommunication wavelengths (1500-1600 nm) were obtained by fitting the transmittance combined with spectroscopic ellipsometry measurement.