Better electrical and optical properties of ITO thin films were demanded for the window layer of CdS/CdTe solar cells. To match that demand, an ion beam sputtering system was used for the deposition of ITO thin films. The substrate temperature and ion beam energy were controlled to deposit high quality ITO thin films in two cases of Ar ion sputtering and Ar+$O_2$ ion sputtering. The microstructure changed from domain structure in ITO deposited by Ar ions to grain structure in ITO deposited by Ar+$O_2$ ions. The lowest resistivity of ITO films was $1.5 imes10^{-4}{Omega}cm$ at $100^{circ}C$ substrate temperature in case of Ar ions sputtering. Transmittance in the visible range was over 80% above $100^{circ}C$ substrate temperature.