Disclosed is a method for annealing a silicon thin film in a substrate in which an insulation layer and the silicon thin film are subsequently formed. The method includes heating or preheating the silicon thin film within a temperature range at which the substrate is not transformed during the process so as to generate an intrinsic carrier therein, thereby lowering a resistance to a value at which Joule heating is possible; and applying an electric field to the preheated silicon thin film so as to induce Joule heating by means of movement of the carrier, thereby conducting crystallization, eliminating crystal defects, and ensuring crystal growth. When using the method, Joule heating is selectively induced to a-Si thin film, a-Si/Poly-Si thin film or a Poly-Si thin film according to the preheating condition, thereby making a Poly-Si thin film of good quality within a very short time without damaging the substrate.