A novel technique for Si epitaxial lateral overgrowth, called tunnel epitaxy, is proposed and demonstrated. In this technique, lateral epitaxy proceeds in the tunnel between the upper and lower SiO2layers. Limitation of film thickness is achieved, as is a high lateral/vertical growth ratio of approximately 7. Lateral growth distance is limited by clogging of the gas injection window by polycrystalline Si grain growth. Rough surface morphology and twin boundary defects were observed in the fabricated silicon‐on‐insulator. The origins of and possible solutions for these problems are discussed.