A thin film transistor includes an active layer on a substrate, a gate electrode configured to be spaced from the active layer and partially overlapped with the active layer, and a gate insulating layer, at least a part of the gate insulating layer being disposed between the active layer and the gate electrode, wherein the gate insulating layer includes a first gate insulating layer between the active layer and the gate electrode, and a second gate insulating layer configured to have a dielectric constant (k) which is different from a dielectric constant of the first gate insulating layer, and disposed in a same layer as the first gate insulating layer, and wherein at least a part of the second gate insulating layer is disposed between the active layer and the gate electrode.