The chemical structures of SiO2/Si interfaces were studied by photoelectron spectroscopy using high-brilliance soft X-ray with photon energy ranging from 500 to 1500 eV at Super Photon ring 8 GeV(SPring-8) and it is able to probe a depth of about 1.2 to 3 nm with energy resolution of 100 meV. On the other hand, high-brilliance hard X-ray with photon energy ranging from 6 to 10 keV is able to probe a depth of about 8.5 to 12.5 nm with energy resolution of 100 meV. Hard photoelectron spectroscopy are particularly useful for studying the composition and the chemical structure of transition layer at high-k dielectric/silicon interface.