A polycrystalline silicon layer, a flat panel display using the polycrystalline silicon layer, and methods of fabricating the same are provided. An amorphous silicon layer is formed on a substrate. A first pattern layer, a second pattern layer, and a metal catalyst layer are formed on the amorphous silicon layer. The first pattern layer and the second pattern layer are formed to define a region of at least 400 μm2 within which a metal catalyst of the metal catalyst layer is diffused into the amorphous silicon layer. A seed region is crystallized by the diffused metal catalyst. After a crystallization region is grown from the seed region, a semiconductor layer is formed on the crystallization region, so as to fabricate a thin film transistor with excellent characteristics. Using this, a flat panel display is fabricated.