We investigated the atomic structure of the SiO2/Si interface and the initial oxidation process of Si surfaces using our developed large-scale atomistic simulation technique for Si, O mixed systems. We constructed large-scale SiO2/Si(0 0 1) interface models (now up to 12,536 atoms in size) by inserting O atoms into Si-Si bonds in crystalline Si substrates from the surface of the models. The resulting SiO2/Si Models exhibited a compressively strained oxide region near the interface, and reproduced X-ray diffraction peaks compatible with experimental results. Using the large-scale modeling technique, we simulated an atomistic oxidation process where the O atoms were introduced into the Si substrate in one by one so as to minimize the strain energy caused by the insertion of the O atoms. A mostly abrupt change in the composition at the SiO2/Si interface was reproduced in this energetic scheme, though the oxidation did not proceed layer by layer as previously reported by many other reports. We found out that the layer-by-layer oxidation phenomenon can be explained by the kinetics of oxidants arriving at the interface through the oxide film. (C) 2004 Elsevier B.V. All rights reserved.