The structure of silicon‐on‐insulator formed by oxygen implantation at 150 keV with a dose of 1.6×1018cm−2is studied by high resolution transmission electron microscopy. Polyhedral oxygen precipitates are observed both in the top Si layer and in the substrate after 1150 °C annealing. The oxygen precipitates in the top Si layer coarsen at 1250 °C, but no precipitate can be found in the substrate at this temperature. A layer of polycrystalline silicon (polysilicon) exists near the top Si/buried oxide interface. Silicon crystals in the polysilicon layer also coarsen when the annealing temperature is changed from 1150 to 1250 °C. At 1150 °C, the buried oxide/substrate interface has many Si lamellas roughly parallel to the {100} wafer surface. The lamellar structure is broken up after 1250 °C annealing and is replaced by islandlike crystals which are strongly faceted on {100} planes. Defects are present in Si microcrystallites in both the polysilicon layer and the islandlike particles.