A thin film transistor is provided. The thin film transistor includes an oxide semiconductor layer on a substrate, a gate electrode insulated from the oxide semiconductor layer to overlap at least a portion of the oxide semiconductor layer, a source electrode connected to the oxide semiconductor layer, and a drain electrode spaced apart from the source electrode and connected to the oxide semiconductor layer. The oxide semiconductor layer includes a first oxide semiconductor layer on the substrate and a second oxide semiconductor layer on the first oxide semiconductor layer, the first oxide semiconductor layer includes nitrogen of 1 at % to 5 at % concentration with respect to number of atoms, and the second oxide semiconductor layer has a nitrogen concentration which is lower than a nitrogen concentration of the first oxide semiconductor layer and a gradient of the nitrogen concentration such that the nitrogen concentration is lowered in a direction closer to the gate electrode.