A method of forming a polycrystalline silicon layer, a thin film transistor (TFT), an organic light emitting diode (OLED) display device having the same, and methods of fabricating the same. The method of forming a polycrystalline silicon layer includes providing a substrate, forming a buffer layer on the substrate, forming an amorphous silicon layer on the buffer layer, forming a groove in the amorphous silicon layer, forming a capping layer on the amorphous silicon layer, forming a metal catalyst layer on the capping layer, and annealing the substrate and crystallizing the amorphous silicon layer into a polycrystalline silicon layer.