A new analytical method for thin Si layer is proposed that uses low temperature oxidation and chemical analysis. The thin Si layer of silicon-on-insulator wafers is oxidized at a low temperature by electron cyclotron resonance oxygen plasma. The oxide is analyzed by vapor phase decomposition method and atomic absorption spectrometry. This approach makes it possible to analyze metallic impurities without the diffusion of contaminants because the oxidation is carried out at a low temperature. The results reveal that metallic impurity concentrations below 1010atoms/cm2can be detected with a depth resolution of 4 nm.