The effects of the $O_2$ concentration in the Ar sputtering gas and the substrate temperature on the structural and optical properties of 3mol% YSZ and 8mol% YSZ thin films deposited by RF magnetron sputtering process were investigated. The films were observed to have various crystal structures with different compositions in accordance with the materials of the target. The size of fine grain-like particles increased with increasing $O_2$ concentration in the sputtering gas in the case of 8mol% YSZ while decreased in the case of 3mol% YSZ. The averate transmission of 8mol% YSZ, despite of thicker thickness, was higher than that of 3mol% YSZ. Also, the transmission spectra of 8mol% YSZ films were not strongly influenced by the substrate temperature.