A thin film transistor (TFT), including a substrate, a gate electrode on the substrate, an oxide semiconductor layer including a channel region, a source region, and a drain region, a gate insulating layer between the gate electrode and the oxide semiconductor layer, and source and drain electrodes in contact with the source and drain regions of the oxide semiconductor layer, respectively, wherein the oxide semiconductor layer has a GaInZnO (GIZO) bilayer structure including a lower layer and an upper layer, and the upper layer has a different indium (In) concentration than the lower layer.