A thin film transistor is provided. A thin film transistor according to an exemplary embodiment of the present invention includes: a substrate; a gate line disposed on the substrate and including a gate electrode; a semiconductor layer disposed on the substrate and including at least a portion overlapping the gate electrode; a gate insulating layer disposed between the gate line and the semiconductor layer; and a source electrode and a drain electrode disposed on the substrate and facing each other over a channel region of the semiconductor layer. The gate insulating layer includes a first region and a second region, the first region corresponds to the channel region of the semiconductor layer, the first region is made of a first material, the second region is made of a second material, and the first material and the second material have different atomic number ratios of carbon and silicon.