In this paper, we prepared $Cu(In,Ga)Se_2$ thin films by using co-evaporation method, and analyzed the properties of the thin films. During the thin film preparation process, we confirmed $InGaSe_2$ phase was formed at $400^{circ}C$ in first stage, and also confirmed the thin films showed the vacancy decrease. In second and third stage, we confirmed the density increase of crystalline structure at over $480^{circ}C$ and the formation of $Cu(In_{0.7}Ga_{0.3})Se_2$ phase. As the result of SEM and XRD analysis of the films which were before and after heat-treated, we confirmed the disappearance of $Cu_2Se_2$ and the formation of $Cu(In_{0.7}Ga_{0.3})Se_2$ single phase after the heat-treatment, We, therefore, confirmed the heat-treatment did not affect the absorbency spectra of the thin films.