We performed an experimental study of the effect of the gas phase composition on the growth mechanism of 3C-SiC on Si(100) by atmospheric-pressure vapour phase epitaxy at 1350°C. Silane and propane diluted in hydrogen were used as precursors for the growth. We demonstrate the existence of an equilibrium partial pressure of carbon above the growing surface, which ensures a mirror-like morphology. For too low a carbon partial pressure (C/Si ratio in the gas phase lower than 2.7 with a growth rate of 3 μm h−1), the layer morphology and crystalline quality quickly degrade. For too high a carbon partial pressure (C/Si ratio higher than 5 with the same growth rate), SiC clusters form on the growing layers. We propose a mechanism of formation for these clusters taking into account the interactions between the C and Si species in the hot boundary layer.