Polycrystalline silicon thin films were prepared at high-speed by plasma-enhanced chemical vapor deposition technique at low temperatures using SiCl4 and H2 as source gases. It was found that the grain growth is strongly affected by the relative concentration of different active radicals in the gas-phase space. On the other hand, the relative concentration depends on the deposition conditions. With the decrease of the rf power and the H2/ SiCl4 flow ratio, and the increase in the reaction pressure, the grain size increases. By changing the deposition conditions, variations of the relative concentration were analyzed. It is suggested that the “gas-phase crystalline" is of crucial importance to the grain growth.