A new method was proposed to investigate the transport mechanism of silicon oxidation at 1100 ℃ using H216O/H218O isotopic labeling. The formation and structure of silicon oxide film was analysed. The distribution of 16O and 18O in the oxide film was analysed by means of secondary ion mass spectroscopy (SIMS). The results demonstrate that the oxide film is non-crystalline during the oxidation of silicon in the water vapor and the transport mechanism is substitutional diffusion mechanism.