Optical properties of oxygen-doped Ge-Sb-Te thin films prepared by RF-sputtering method in the region of 400～800 nm were studied, including refractive index, extinction coefficient. The results show that optical constants of the Ge-Sb-Te-O films change with oxygen content and heat-treatment. XRD spectra of the films with different oxygen content in the as-deposited and heat-treated states show that the films changed from amorphous to crystalline states due to heat-treatment. The effect of the strain field induced by oxygen-doping on optical properties is discussed.