A method of fabricating a polycrystalline silicon (poly-Si) layer includes providing a substrate, forming an amorphous silicon (a-Si) layer on the substrate, forming a thermal oxide layer to a thickness of about 10 to 50 Å on the a-Si layer, forming a metal catalyst layer on the thermal oxide layer, and annealing the substrate to crystallize the a-Si layer into the poly-Si layer using a metal catalyst of the metal catalyst layer. Thus, the a-Si layer can be crystallized into a poly-Si layer by a super grain silicon (SGS) crystallization method. Also, the thermal oxide layer may be formed during the dehydrogenation of the a-Si layer so that an additional process of forming a capping layer required for the SGS crystallization method can be omitted, thereby simplifying the fabrication process.