A thin film transistor includes a substrate, a buffer layer on the substrate, a semiconductor layer including source/drain regions and a channel region on the buffer layer, a gate insulating layer corresponding to the channel region, a gate electrode corresponding to the channel region, and source/drain electrodes electrically connected to the semiconductor layer. A polysilicon layer of the channel region may include only a low angle grain boundary, and a high angle grain boundary may be disposed in a region of the semiconductor layer that is apart from the channel region.