Metal nanoparticles in application for silicon based single electron devices are formed in a thin SiO2 layer such as gate oxide layer on silicon substrate. The TEM observation is reliable but takes much effort and a long time for obtaining information about state of nanoparticles, i.e. particle size, distribution depth. Here, optical reflection property from multiple layer structure on Si was studied by experiments and with theoretical analysis. We proposed a calculation method of the reflectance from Au implanted 50-nm-thick SiO2 film on Si substrate with three-layers model of the top SiO2 a composite layer with Au nanoparticles, and bottom SiO2 layer. The experimentally obtained reflectance spectra from a sample at 35 keV-1 x 10(16) ions/cm(2) are well fitted by the proposed analysis of changing particle size and distribution depth based on the depth profiles of implanted atoms. (c) 2004 Elsevier B.V. All rights reserved.