Transparent conductiong films of cadmium-tin oxide were prepared by rf reactive sputtering from a Cd-Sn alloy target in an Ar-O2 atmosphere. The structure of the films was examined by X-ray diffraction. The electrical and optical properties of the films were found to depend on the oxygen concentration in the gas mixture and the substrate temperature as well as the post-deposition heat treatment. The lowest resistivity obtained was 1.74×10-6Ω·cm and the highest optical transmission was 95% over the visible region. For the films deposited at substrate temperature of 400℃ in an Ar-O2 atmosphere of 6% O2, heat treatment increased the optical gap energy from 2.37eV to 2.64eV.