Heteroepitaxial growth of 3C-SiC on Si substrates by gas source molecular beam epitaxy was investigated. Both Si(001) and Si(111) surfaces were carbonized using a C2H2 gas molecular beam to convert the surface region into single crystalline 3C-SiC prior to crystal growth. The supply of C2H2 was started at 400°C, and the substrate temperature was raised at a rate of 7°C/min. An amorphous-like layer was observed at 870°C. Raising the temperature at a rate of 2°C/min from 870 to 970°C, a single crystalline 3C-SiC layer was obtained. In the case of Si(001), increase of C2H2 supply resulted in improvement of crystallinity, because of a thin (∼50 Å) 3C-SiC layer formed at an early stage of carbonization, which prevented outdiffusion of Si atoms. The thickness of the 3C-SiC layer did not increase for prolonged time of carbonization after formation of the thin layer. In the case of Si(111), the increase of C2H2 supply resulted in a thicker layer of 3C-SiC with a rough surface, because the channels of Si outdiffusion were not sealed off. Epitaxial growth of 3C-SiC was carried out using alternate supply of Si2H6 and C2H2 gas molecular beams on 3C-SiC layers obtained by carbonization under optimum conditions. Single crystalline 3C-SiC layers were obtained at 1075°C, both on (001) and (111) surfaces.