We present the results of optical properties of multi layer thin films as in the media of phase change optical disk data storage. Reflectance and optical contrast of multi layer thin films increased rapidly between 100 °C and 150 °C. Moreover, optical contrasts at different wavelength were also studied. The refractive index and the optical band gap decreased, while the extinction coefficient increases as the crystallization occurs. The Eg opt of crystalline thin film was ∼0.6 eV lower than that of amorphous thin film. Eg opt decreased as the number of stacked layer increased.