The optical properties of Si/SiO2 interface formed at 850 degrees C oxidation of silicon were studied by using spectroscopic ellipsometry in the range of 280-632.8 nm (4.43-1.95 eV). The results indicate a 1.0 nm thick highly stressed interface with a physical composition of Si-0.6(SiO2)(0.4) Oxidation induced stress causes changes in the optical constants of Si substrate in the strained region beneath Si/SiO2 interface. The thickness of this optically nonhomogeneous region is estimated to be about 5 nm.