The effects of annealing on undoped a-Si:H films were studied. Films of a-Si:H were prepared by mercury-sensitized photochemical vapor deposition, and were annealed immediately. The hydrogen configuration was altered in the annealing process below 200°C, although films were prepared at a substrate temperature of 230°C. At the same time, electron drift mobility greatly increased. These observations indicate a reduction in the number of tail states in the a-Si:H band-gap. Thus the annealing below the substrate temperature clearly influences not only the bonded-hydrogen atoms but also the Si-Si network in the films to some extent.