A thin film transistor, a method of fabricating the thin film transistor, and an organic light emitting diode (OLED) display device including the thin film transistor, the thin film transistor including: a substrate; a buffer layer formed on the substrate; a first semiconductor layer disposed on the buffer layer; a second semiconductor layer disposed on the first semiconductor layer, which is larger than the first semiconductor layer; a gate electrode insulated from the first semiconductor layer and the second semiconductor layer; a gate insulating layer to insulate the gate electrode from the first semiconductor layer and the second semiconductor layer; source and drain electrodes insulated from the gate electrode and connected to the second semiconductor layer; an insulating layer disposed on the source and drain electrodes, and an organic light emitting diode connected to one of the source and drain electrodes.