A simple technique is presented for evaluating defect profiles in ultrathin Si films on buried SiO2 formed by implanted oxygen. A combination of thinning by sacrificial oxidation and epitaxial film growth by UHV-CVD is used. By measuring the defect density of the epitaxial film with respect to the thickness prior to epitaxial growth, the profile of the initial defect density can easily be evaluated. This technique is applied to evaluate the Si on insulator structure fabricated by state-of-the-art technique, in which low dose oxygen implantation (∼4X1017 cm-2) and high temperature internal oxidation processes are used. The defect density at the surface of the film is 250 cm-2. However, as the buried interface is approached, the defect density increases. The defect density at 20 nm from the buried interface is as high as 6X105 cm-2. A defect generation mechanism is also discussed.