Hydrogenated nanocrystalline silicon film have been deposited by RF sputtering in a purl hydrogen plasma at a substrate temperature of 150 degrees C. The unintentionally grown amorphous buffer layer was almost completely crystallized at an annealing temperature as low as 150 degrees C. By combining infrared absorption measurements and electron microscopy observations, the critical role of dihydride species, Si-H-2, in the crystallization process have been clearly evidenced and discussed.