Behavior of gold (Au) on n-type Si(100) surface dipped in Au aqueous solution and thermally oxidized have been studied using by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), alternating current surface photovoltage (AC SPV) techniques and ellipsometer. AFM and XPS analyses identified Au to exist at both the top of SiO₂ and SiO₂/Si interface as a cluster. Au/n-type Si Schottky-barrier causes an occurrence of frequency-dependent AC SPV. The frequency-dependent AC SPV still appeared in Au-contaminated (∼2×10¹⁵ atoms/cm²) and thermally oxidized Si surfaces at between 550 and 700^oC, demonstrating that Si surface was weakly inverted. This result indicates that Au/n-type Si Schottky-barrier is formed at SiO₂/Si interface even after thermal oxidation. The Au at SiO₂/Si interface diffused into bulk Si as temperature increased higher than 750^oC, resulting in drastic reduction of AC SPV. On oxidation kinetics at between 750 and 900^oC, Au is thought to act as catalyst to promote SiO₂ growth at the Si surface, resulting in the enhanced oxidation.