It was investigated and found that the enhanced diffusion of arsenic and phosphorus in silicon by thermal oxidation is dependent on the crystal orientation of a substrate, diffusion temperature, and time. Impurities are prediffused into a silicon substrate from impurity‐silica film, and then subjected to a drive‐in diffusion in oxidizing (dry ) and inert (dry ) atmosphere. Diffusion coefficients are determined by adopting best fitted parameters in the numerical simulation of the experiments. The diffusion coefficients of arsenic and phosphorus are enhanced by thermal oxidation of silicon. This enhancement measured in (100) silicon is greater than that in (111) silicon. The diffusion coefficients obtained in oxidizing atmosphere can be expressed as a function of diffusion temperature and time. From the form of this equation, it is considered that arsenic and phosphorus diffuse by the dual mechanism (vacancy and interstitialcy mechanism). The enhancement increases in order of arsenic, phosphorus, and boron.