We used X-ray diffraction to quantitatively measure the strain induced by the local oxidation of silicon in thin-film silicon-on-insulator (SOI) wafers. In the samples of bonded SOI wafers thinned by plasma-assisted chemical etching, the reflection peaks of the top Si layer and the base Si substrate could be measured independently because the orientation of these lattice planes differs slightly. Moreover, the strain near the surface region could be measured because the top Si layer is only 200-nm thick. We show that the strain in the SOI wafers is more than one order of magnitude larger than that in the bulk wafers when the device region is fully isolated by the surrounding SiO₂.