Abstract: Method of forming a uniform polycrystalline semiconductor thin film by laser annealing. The method is started with preparing a substrate having an insulating layer which has a relatively low thermal conductivity and a thickness of more than 20 nm. Then, an amorphous silicon thin film having a relatively high thermal conductivity is formed to a thickness of less than 35 nm on the insulating layer. Thereafter, the amorphous silicon thin film is irradiated with laser beam to impart thermal energy to the film. In this way, the amorphous film is converted into a polysilicon thin film. Since the thickness of the amorphous silicon film is less than 35 nm, polysilicons having uniform grain diameters can be grown.