A method of crystallizing amorphous silicon, wherein the method includes supplying nanoparticles over a surface of an amorphous silicon layer; intermittently melting nanoparticles that reach the surface of the amorphous silicon layer while supplying the nanoparticles; and cooling the amorphous silicon layer to grow crystals using unmolten nanoparticles as crystal seeds, thereby forming a polysilicon layer. Externally supplied nanoparticles are used as crystal seeds to crystallize an amorphous silicon layer so that large grains can be formed. Accordingly, since the number and size of nanoparticles may be controlled, the size and arrangement of grains may also be controlled.