Indium tin oxide ( ITO ) thin films ( 100 ± 10 nm) were deposited on PC ( polycarbonate ) and glass substrates by rf( radio-frequency ) magnetron sputtering. The oxygen content of the ITO films was changed by variation of the sputtering gas composition. All the other deposition parameters were kept constant. The sheet resistance, optical transmittance and microstructure of ITO films were investigated using a four-point probe, spectrophotometer, X- ray diffractometer( XRD ) and atomic force microscope ( AFM ). Sheet resistances for the ITO films with optical transmittance more than 75% on PC substrates varied from 40 Ω/ cm2 to more than 104 Ω/cm2 with increasing oxygen partial pressure from 0 to about 2% . The same tendency of sheet resistances increasing with increasing oxygen partial pressure was observed on glass substrates. The X-ray diffraction data indicated polycrystalline films with grain orientations predominantly along (440) and (422) directions. The intensities of (440) and (422) peaks increased slightly with the increase of oxygen partial pressure both on PC and glass substrates. The AFM images show that the ITO films on PC substrates were dense and uniform. The average grain size of the films was about 40nm.