Indium-tin-oxide (ITO) thin films, used as transparent conductive electrodes in various optoelectronics fields, were prepared via RF sputtering at dierent substrate temperatures. The eects of substrate temperature on the optical properties were investigated. The structures and the optical constants of the films were successfully analyzed using the classical formula. The microstructures and the resistances of ITO films were strongly influenced by the substrate temperature, and the optical properties of the films significantly changed with variations in the microstructure and the resistance induced by increasing the substrate temperature. The reduction of the resistance, indicating an increase in the free electron concentration, induced a Burstein-Moss shift in the UV region and a Drude absorption in the near infrared (NIR) region.