Growth of 3C-SiC on Si substrates has been carried out by atmospheric pressure chemical vapor deposition using hexamethyldisilane as the source and a H-2/Ar gas mixture as the carrier gas. No separate carbonization step was used. Single crystalline 3C-SiC on Si(001) was grown using a two-step growth technique. For Si(001) substrates, the nucleation stage was optimum in the temperature range 1250-1300 degrees C for a growth time of approximately 2 min. A high concentration of H-2 (12%) in the carrier gas favored epitaxial growth on these substrates, On the other hand, the optimum temperature range for the nucleation stage on Si(111) was 1150-1200 degrees C and the optimum time for the nucleation stage was longer (similar to 5 min). Additionally, it was possible to achieve single crystallinity on Si(111) substrates with a lower H-2 content (8%) in the carrier gas. The second stage of growth (at higher temperature of similar to 1380 degrees C) was identical for both substrate orientations. The difference in the nucleation stage can be attributed to the lower surface free energy of the Si(111) substrate resulting in a smaller critical cluster size. Additionally, the presence of microtwins in the nucleated clusters enhances growth of the SiC(111) oriented nuclei on Si(001) substrates.