A thin film transistor includes a substrate, a gate electrode, a buffer layer, a gate insulating layer, an active layer, an etching stop layer, a source electrode and a drain electrode. The gate electrode is formed on the substrate. The buffer layer partially covers both side portions of the gate electrode. The gate insulating layer covers the gate electrode and the buffer layer. The active layer is formed on the gate insulating layer. The etching stop layer is formed on the active layer, and has a first opening and a second opening on the active layer. The source electrode is formed on the etching stop layer, and contacts with the active layer through the first opening. The drain electrode is formed on the etching stop layer, and is contacted with the active layer through the second opening.