The limiting conditions are presented of selective epitaxial growth (SEG) on a SiO2‐patterned Si (001) substrate for Si gas‐source molecular‐beam epitaxy by use of 100% Si2H6. In the initial stage of growth, epitaxial Si was selectively grown on a Si surface in the wide temperature range of 500–850 °C. On the other hand, polycrystalline Si nucleation on a SiO2surface was intimately related to the total volume of supply gas. At a substrate temperature of 700 °C and a Si2H6flow rate of 60 sccm, a SEG layer could be deposited at a rate as high as 645 A˚/min.