In a method of manufacturing a thin film transistor, a gate electrode is formed on a first surface of a base substrate, a oxide semiconductor layer, insulation layer and photo resist layer are formed an the fast surface of the base substrate having the gate electrode. The insulation layer and the oxide semiconductor layer are patterned using a first photo resist pattern to form an etch-stopper and an active pattern. A source and a drain electrode are formed on the base substrate having the active pattern and the etch-stopper, the source electrode and the drain electrode are overlapped with both ends of the etch-stopper and spaced apart from each other. Therefore, a manufacturing cost may be decreased by omitting a mask when forming the active pattern and the etch-stopper.