A light having a pulse frequency higher than about 300Hz is generated. The light is irradiated on an amorphous silicon thin film for a predetermined time period to form an initial polysilicon crystal. The light is transported in a predetermined direction to grow the initial polysilicon crystal. A laser beam having a decreased output energy is irradiated on the amorphous silicon thin film to crystallize the amorphous silicon thin film to a polysilicon thin film so that the load of an apparatus for generating the laser beam is decreased, and the lifetime of the apparatus for generating the laser beam increases.
