We report the oxidation characteristics for Si nanostructures. Synchrotron radiation Si 1s X-ray photoemission spectroscopy (XPS) is used to explain the oxidation process and the modification of the electronic structures of Si. Si films of different thicknesses were grown on a graphite surface. Before oxidation, several photoemission components are observed in thin Si films, such as 2 or 4 Angstrom films. The components observed at the higher binding energy regions are identified as Si nanostructures. After oxidation, photoemission results show that the oxidation process is thickness dependent. Quantitatively, in 2 and 4 Angstrom films, the photoemission signals from oxide phase are 18 and 30%, respectively of total intensity. The present result suggests that the higher binding energy components, i.e., Si nanostructures play some significant role during the oxidation, where the reactivity of nanostructures towards oxygen is relatively low or negligible, and nanostructures impose some restrictions on the oxidation of the film. (C) 2004 Elsevier B.V. All rights reserved.