The In doped ZnO(ZnO:In)thin films sensitive to NH₃ gas were prepared by the double layer depositions of In film by vacuum evaporation and ZnO film by rf magnetron sputtering method onto a SiO₂/Si wafer substrate, and subsequent heat treatment process. The structural and electrical characteristics of the ZnO:In thin films were studied as a function of heat treatment temperature by x-ray diffraction, scanning electron microscope and 4 point probing method. And the dependence of the sensitivity, the selectivity and the time response of the thin films on heat treatment temperature was investigated. The thin film heat-treated at 400 ℃ showed the highest sensitivity of 140 % at an operating temperature of 300 ℃. The sensitivity towards CO, NO_x gases observed in the same temperature.