A thin film transistor includes a gate electrode, a gate insulating layer, an oxide semiconductor layer, an oxide buffer layer, a protective layer, and source and drain electrodes. The gate electrode is formed on a substrate. The gate insulating layer is formed on the substrate. The oxide semiconductor layer is formed on the gate insulating layer and includes a source, a channel and a drain region. The oxide buffer layer is formed on the oxide semiconductor layer, and has a carrier concentration lower than that of the oxide semiconductor layer. The protective layer is formed on the oxide buffer layer and the gate insulating layer, and has contact holes formed therein so that the oxide buffer layer in the source and drain regions are exposed therethrough. The source and drain electrodes are coupled with the oxide buffer layer in the source and drain regions through the contact holes.