Si epitaxial layers were selectively grown on local oxidation of silicon patterned Si (100) substrates by the cold wall ultrahigh vacuum chemical vapor deposition under various growth conditions. The isotropic/anisotropic growth behavior and the faceting morphology of Si epitaxial layers were systematically investigated. As the growth temperature increased and the Si2H6 flow rate decreased, the lateral overgrowth of Si was reduced, and subsequently the anisotropic selective epitaxial growth (SEG) of Si was enhanced. Depending on growth conditions, the lateral overgrowth was not initiated until the layer thickness exceeded a critical value, and the degree of lateral overgrowth was changed with the layer thickness. These observations strongly imply that the mass transport and accumulation processes on facet surfaces play an important role in the SEG morphological change. Taking both surface mass transport and free energy change into account, a model is proposed to explain our experimental observations, and the detailed discussion is provided. Finally, we confirmed the proposed model through the experimental and theoretical analysis of the Si growth rate change on the (111) facet plane with facet length, which highlights the strong dependence of the mass transport and accumulation processes on the difference between facet length and surface diffusion length. © 2004 American Vacuum Society.