To analyze the structure of the SiO2/Si interface, we developed a method that uses spectroscopic ellipsometry and x-ray reflectometry simultaneously. We applied this method to the characterization Of SiO2 grown in dry oxygen. We found that the thickness of the interface layer showed no significant dependence on the oxidation temperature and oxide film thickness. The mean value of the interface layer density was 2.31 g/cm(3). clearly higher than that of the upper layer, which was 2.28 g/cm(3). Furthermore, we found that the distribution of the interface layer density in the 6 in. wafer was similar to the distribution of the interface state density, D-it, which was obtained by the charge-pumping method using metal-oxide-semiconductor field-effect transistors with SiO2 formed in the same oxidation process. This fact strongly Suggests that the density of the interface layer is correlated closely with the electrical characteristics Of the SiO2/Si interface.