We investigated a thermally deposited amorphous silicon (TAS) film before and after annealing. We used monosilane (SiH4) or disilane (Si2H6) as the Si source gas at a deposition pressure of 0.1 to 0.5 Torr. The activation energy of SiH4 deposition was 1.7 eV (560 to 600-degrees-C) and that of Si2H6 was 0.6 eV (510 to 570-degrees-C). From TEM observation, the TAS film from 450-degrees-C Si2H6 deposition was completely amorphous without crystals and had a smooth surface. Film deposited at 560-degrees-C had a few crystals at the Si/SiO2 interface and had a few bumps on the Si surface. After annealing, the mean grain size of 450-degrees-C-Si2H6 film was about 2 mum and that of 560-degrees-C-SiH4 film was about 0.3 mum. We also evaluated the crystallinity by XRD and Raman spectroscopy. Films deposited at lower temperatures after annealing showed strong <111>-orientations and high crystal qualities.