In this paper the nucleation mechanisms for SiC thin film growth are studied by rapid thermal CVD and atomic force microscopy (AFM). The nucleation mode was found to be strongly dependent on the hydrocarbon partial pressure in the gas stream, for a fixed reaction time. In the case of three-dimensional (island) growth at low precursor concentration, cross-sectional SEM micrographs indicate that no voids are present at the center of each nucleus (or island). Voids begin to form when two neighboring nuclei come in contact. AFM has shown that trenches are present in the Si substrate around each isolated nucleus. The trench depth increases with the diameter of the island. AFM analysis of films grown for a nominal reaction time of 1 sec at different propane concentrations indicates that: (1) SiC grain size and surface roughness decreases with increasing propane concentration; (2) SiC grain density increases with increasing propane concentration. Based on the above evidence, the following nucleation mechanism is proposed: (a) the initial nucleation density is determined by the precursor concentration in the reaction gas; b) each nucleus grows larger, both laterally and vertically, by consuming Si around it; (c) voids are formed when nuclei grow large enough to come in contact, and not at the original nucleation sites.