Our investigation of impurity diffusion in Si has clarified diffusion mechanisms and the point defect structure for the material. Thermal equilibrium between interstitials and vacancies in high temperature Si was clarified through observation of oxidation-retarded diffusion (ORD) of Sb in Si. Boron, Phosphorus, Aluminum and Galium were found to diffuse mainly due to an interstitialcy mechanism, while Sb did by a vacancy mechanism. Moreover, the atomic number in Si crystal compared to that at lattice sites was seen to play a dominant role in determining point defect concentration. Oxidation of Si and nitridation of SiO2 on Si lead to an increase in the atomic number of Si crystals and cause an increase in interstitial concentration as well as a decrease in vacancy concentration. On the other hand, nitridation of Si leads to a decrease in atomic number and causes vacancy super-saturation as well as interstitial under-saturation.