We have studied the polycrystalline growth of Si prepared by a reactive gas flow of SiH4 and F2. In a very early stage of the film growth on the amorphous substrates, a thin layer of the amorphous phase was deposited even in the favorable condition for the crystal growth, whose thickness was determined by the growth condition and reached to 200–300nm in a certain condition. The crystalline phase consisted of closely packed small grains perpendicular to the substrate, 200–500nm in diameter, each of which displayed a tooth-like shape with fine structures, indicating a formation of the crystal nuclei in the amorphous phase and their growth to the grains. The generation probability of the nuclei was not uniform in the amorphous phase, high at the region far from the substrate, and very sensitive to the growth temperature, judging from cross sectional TEM micrographs of the films. These suggest that the network propagation of Si in which fluorine, participates the chemical process in the vicinity of the growing surface enhances a formation of the long range ordering in the network, resulting in the generation of the crystal nuclei.