Nanocrystalline diamond films have been deposited on Si substrates by microwave plasma-enhanced chemical-vapor-deposition (MPECVD) method. The microstructure and morphology of the as-deposited carbon films are found to be strongly influenced by substrate pretreatment methods. Under higher methane concentration conditions, nanocrystalline diamond films could be grown on Si substrates which were ultrasonically scratched both by diamond and SIC powders, but their surface topographies were rather different. On the substrates which were mechanically abraded by the Al2O3 grits, nano-diamond particles are formed in the amorphous matrix, that is considered to be the diamond-like carbon (DLC). The pretreatment effects on the formation of nano-diamonds are discussed related to the nucleation processes. The study also indicates that by a two-step (high methane concentration followed by a low methane concentration) growth process, even on the substrates scratched by the low-cost nondiamond abrasives, fabrication of polycrystalline diamond films with high grain density is possible.