Provided is a method of crystallizing an amorphous silicon thin film transistor and a method of fabricating a polycrystalline thin film transistor using the same, in which the polycrystalline thin film transistor indicating leakage current characteristics of a level that is applicable for active matrix organic light emitting diode displays (AMOLEDs) can be manufactured by using a silicide seed induced lateral crystallization (SILC) method. The amorphous silicon thin film transistor crystallizing method includes the steps of: forming an amorphous silicon layer on a substrate; forming an active region by patterning the amorphous silicon layer; forming a crystallization induced metal layer in both a source region and a drain region that are placed on both side ends of the active region; forming a number of dot-shaped metal silicide seeds on the surfaces of the source region and the drain region made of amorphous silicon by removing the crystallization induced metal layer; and crystallizing the active region formed of the amorphous silicon layer by heat-treating the substrate by using the metal silicide seeds as crystallization seeds.