Abstract: Single-crystal silicon carbide film were grown on sapphire substrate with a Nitride buffer-layer using APCVD （Atmospheric Pressure Chemical Vapor Deposition）. Silicon-vapor crystallization process was discussed. It is known that a reliable control of transport of Si-and C-containing species to the growth region and C/Si ratio over the wafer surface is of crucial importance to surface morphology and doping level of SiC epitaxial layer. The results show that gas phase crystallization of Si-droplets are critical to control the C/Si ratio over the growing region. A simple set of chemical reactions is provided to simulate precursors and gas spices distribution in quartz tube. SEM observation also verifys that silicon-vapor crystallization and surface absorption of Si-droplet do occur in growing process of SiC. Gas phase crystallization of Si-droplets also give a good explanation to the relations between epi-layer growth rate and precursors’ flow rate.