We study the impact of the nucleation step on the final crystalline quality of 3C-SiC heteroepitaxial films grown on (111) and (100) oriented silicon substrates by low pressure chemical vapor deposition. The evolution of both the structural and morphological properties of 3C-SiC epilayers in dependence on the only nucleation parameters (propane flow rate and duration of the process) are investigated by means of x-ray diffraction, scanning electron, atomic force, and optical microscopies. At first, we show how the formation of interfacial voids is controlled by the experimental parameters, as previously reported, and we correlate the density of voids with the substrate sealing by using an analytical model developed by V. Cimalla etal [Mater. Sci. Eng., B 46, 190 (1997)]. We show that the nucleation stage produces a more dense buffer layer in case of (111) substrates. Further, we investigate the impact of the nucleation parameters on the crystalline quality of 3C-SiC epilayers. Within our experimental setup, the crystalline quality of (100) oriented 3C-SiC films is more rapidly evolving than (111) films for low propane contents (0.025%–0.05% in hydrogen), whereas a common degradation of the crystalline quality is reported for both cases for the higher propane contents. In parallel, we investigate the morphological features of the epilayers. The (111) oriented epilayers are well coalesced irrespectively of the nucleation condition, contrarily to the (100) films. Finally, for both orientations we report on the dependence of the formation of double positioning domains (twins) on the nucleation conditions. Such defects can be suppressed within (111) films but not within (100) films. We highlight the role of the substrate sealing and discuss in what extent it can be responsible of the observations by reducing the contribution of th- e silicon outdiffusing and by allowing a more pronounced two-dimensional growth mode for (111) oriented 3C-SiC films.