A double‐heteroepitaxial Si/&ggr;‐Al2O3/Si structure was realized. An epitaxial (100)Si layer with high quality was successfully grown on a (100)&ggr;‐Al2O3/(100)Si substrate by Si2H6gas‐source molecular beam epitaxy at substrate temperatures between 700 and 800 °C. The &ggr;‐Al2O3/Si substrate was fabricated by low‐pressure chemical vapor deposition with Al(CH3)3and N2O gases. The reflection high‐energy electron diffraction patterns of the 3000‐A˚‐thick Si epitaxial layer indicated streaked 2×1 patterns. This Si film had a mirror‐like surface, and smooth surface morphology was observed from replica electron micrographs. From the Auger depth profile of the epitaxial layers, it was found that the double‐heteroepitaxial Si/Al2O3/Si structure had a sharper interface between Al2O3and the epitaxial Si film due to the low growth temperatures.