A method of forming a polycrystalline silicon active layer for use in a thin film transistor is provided. The method includes forming a buffer layer over a substrate, forming an amorphous silicon layer over the buffer layer, applying a catalytic metal to a surface of the amorphous silicon layer, crystallizing the amorphous silicon layer having the catalytic metal thereon into a polycrystalline silicon layer, forming an island pattern on the polycrystalline silicon layer, thereby defining an active region underneath in the polycrystalline silicon layer, applying n-type ions to the polycrystalline silicon layer and then heat-treating the polycrystalline silicon layer to remove the catalytic metal from the active region underneath the island pattern, patterning the polycrystalline silicon layer using the island pattern as a mask to form an active layer, doping a small amount of p-type ions into the active layer to adjust a threshold voltage of the then film transistor to be formed using the active layer, and removing the island pattern from the active layer.