A method of forming polysilicon, a thin film transistor (TFT) using the polysilicon, and a method of fabricating the TFT are disclosed. The method of forming the polysilicon comprises: forming an insulating layer on a substrate; forming a first electrode and a second electrode on the insulating layer; forming at least one heater layer on the insulating layer so as to connect the first electrode and the second electrode; forming an amorphous material layer containing silicon on the heater layer(s); forming a through-hole under the heater layer(s) by etching the insulating layer; and crystallizing the amorphous material layer into a polysilicon layer by applying a voltage between the first electrode and the second electrode so as to heat the heater layer(s).