We propose ozone (O-3) as a reactive gas for producing tin-doped indium oxide (ITO) films by DC reactive magnetron sputtering using a metallic alloy target for the first time. We show that incorporation of O-3 enhanced the reaction with the sputtered metal atoms on the substrate surface at room temperature due to its powerful oxidizing nature. The effect of O-3 in oxygen (O-2) gas on the properties of the ITO films was investigated. It is found that high transparency and low resistivity film can be fabricated at high deposition rates and low substrate temperatures by using ozone in oxygen gas.