This paper presents the dependence of the hydrogen contents and the Si-H bonding structure on crystallization in a-Si: H films. The hydrogen contents decreases dramatically and the bonding from transforms from Si-H to Si-H2 obviously when the grain size increases to about 200±50?. We suggest that the 200±50? is a phase transition point corresponds to transformation from mirco-crystalline-Si: H to poly- crystalline-like modification.