The residual stress in silicon substrates after thermal oxidation was discussed experimentally. Microscopic Raman spectroscopy was used for the stress measurement. It was confirmed that the peak position of the Raman spectra shifted linearly with existing stress. The Ar⁺ laser beam of 1μm diameter was radiated into the silicon substrate and the back scattered light was detected by a photon multiplier. It was found that tensile stress occurred near the silicon surface after the plain oxidation. The stress increased with lowering oxidation temperature and thickening oxide film. However, complicated stress change occurred in the silicon substrate after the local thermal oxidation. The stress change was explained considering the curvature change of the Si/SiO₂ interface during the local thermal oxidation.