In this study, transparent conductive films (TCO) of ZnO:Ga (GZO) were deposited by facing target sputtering to explore the effect of post-annealing on the structural, electrical and optical properties of the films. As deposited films have been annealed to each different temperature condition in various atmosphere environments (air, N-2, Vacuum N-2). In the result, in air atmosphere condition, ZnO:Ga films have lost their characteristics as TCO films, because rapid oxidation occurred over 300 degrees C. But in other atmosphere condition, contacting with oxygen is reduced more, as rapid rise of resistivity has been prevented.