A thin film transistor is provided having an oxide semiconductor as an active layer, a method of manufacturing the thin film transistor and a flat panel display device having the thin film transistor. The thin film transistor includes: a gate electrode formed on a substrate; an oxide semiconductor layer isolated from the gate electrode by a gate insulating layer and including channel, source and drain regions; source and drain electrodes coupled to the source and drain regions, respectively; and an ohmic contact layer interposed between the source and drain regions and the source and drain electrodes. In the TFT, the ohmic contact layer is formed with the oxide semiconductor layer having a carrier concentration higher than those of the source and drain regions.