Indium tin oxide (ITO) thin films were deposited onto glass substrates by RF magnetron sputtering to study variation of grain growth in pure argon and 99% argon plus 1% oxygen at different substrate temperatures. The average grain size increased with the increasing substrate temperature in pure argon. However, in oxygen presence environment the grain growth is limited at above 150°C. The films optoelectronic properties were evaluated. It was found that 200 nm ITO films prepared at 220 °C substrate temperature in pure argon possessed optimum sheet resistance of 10 Ω/sq. The transmittance of ITO films was enhanced with increasing the substrate temperature in pure argon but limited by the presence of excess oxygen.