Abstract: A thin film transistor includes a substrate, an oxide semiconductor layer that is disposed on the substrate, a gate electrode that overlaps with the oxide semiconductor layer, a gate insulating layer that is disposed between the oxide semiconductor layer and the gate electrode, and a source electrode and a drain electrode that at least partially overlap with the oxide semiconductor layer and are spaced from each other. The gate insulating layer includes an oxide including a first material. The oxide semiconductor layer includes an oxide which includes a same material as the first material and a second material, and the source electrode and the drain electrode include an oxide that includes a same material as the second material and a third material, and a grain boundary is not formed on an interface between at least one of the gate insulating layer and the oxide semiconductor layer or between the oxide semiconductor layer, and the source electrode and the drain electrode.