A thin film transistor (TFT) using an oxide semiconductor as an active layer, a method of manufacturing the TFT, and a flat panel display device having the TFT include a gate electrode formed on a substrate; an active layer made of an oxide semiconductor and insulated from the gate electrode by a gate insulating layer; source and drain electrodes coupled to the active layer; and an interfacial stability layer formed on one or both surfaces of the active layer. In the TFT, the interfacial stability layer is formed of an oxide having a band gap of 3.0 to 8.0 eV. Since the interfacial stability layer has the same characteristic as a gate insulating layer and a passivation layer, chemically high interface stability is maintained. Since the interfacial stability layer has a band gap equal to or greater than that of the active layer, charge trapping is physically prevented.