A method of manufacturing a thin film transistor includes: forming an active pattern on a substrate; forming an insulating layer and a gate electrode layer on the active pattern in order; forming a photoresist pattern on the gate electrode layer; forming a preliminary gate electrode by wet etching the gate electrode layer using the photoresist pattern; forming an insulating pattern by dry etching the insulating layer using the photoresist pattern and the preliminary gate electrode; and forming a gate electrode by wet etching a side surface of the preliminary gate electrode using the photoresist pattern.