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 C2H2gas molecular beam to convert the surface region into single crystalline 3C‐SiC prior to crystal growth. The supply of C2H2was 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 C2H2supply resulted in improvement of crystallinity, because of a thin (∼50 A˚) 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 C2H2supply 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 Si2H6and C2H2gas 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.