Optical second-harmonic generation (SHG) and Raman spectroscopy have been applied to investigate surface strain/stress appearing at the interface between Si(111) and thermally grown SiO2 layers. From the frequency shift and spectral broadening of the optical phonon mode of Si(111) covered by a 608-Å-thick oxide layer, a tensile stress of 19 kbar was obtained. The azimuthal distribution of the reflected second-harmonic (SH) signal varies with the thickness of surface oxide. To deduce the strain in the lattice-deformed layer, a simple microscopic theory based upon the bond additivity model was proposed, and an agreement between the results of SHG and Raman spectroscopy was achieved. This study suggests that SHG is a sensitive technique for examining surface stress/strain between two lattice-mismatched layers. Therefore it can be useful for the study of the structure of Si1- x Ge x and many other strained-layer systems.