Cadmium-tin oxide (CTO) films with high conductivity and high visible transmission were prepared by dc reactive sputtering from a Cd-Sn alloy target in an Ar-O_2 atmosphere. The electrical and optical properties of these films were found to depend strongly upon the oxygen concentration in an Ar-O_2 atmosphere during deposition. Films with the lowest resistivity of 2.0×(10)^<-4>Ωcm, the highest transmission of 90% over the visible region have been obtained under the condition of 6% oxygen concentration in an Ar-O_2 atmosphere. The structure of the films were examined by X-ray diffraction. These films exhibited a broad Cd_2SnO_4 (130) peak.