Silicon oxide film, which has been used as a protective and active layer in Si devices, has been fabricated electrochemically at room temperature. The fabrication method is anodization combining a constant current and constant voltage mode. Ethylene glycol was used as an electrolyte. To improve the characteristics of silicon oxide film and also the interfacial property of the film with an Si substrate, the oxide film was annealed under hydrogen atmosphere. The depth profile of an atomic concentration was measured by X‐ray photoelectron spectrum (XPS). The thickness of the oxide film was varied by etching with p‐etchant. The as‐grown film is composed of nonstoichiometric silicon oxide of sub‐oxides, SiO2, SiO, and Si2O3, which are nonuniformly present near the Si‐oxide interface up to 40 Å from the Si surface. The remainder of the oxide is stoichiometric amorphous SiO2. After annealing, the amount of sub‐oxides decreases.