Disclosed are a thin film transistor having an oxide semiconductor layer which is applicable to a flat display device requiring high-speed driving due to ultra-high definition, a gate driver including the same, and a display device including the same. The thin film transistor includes a first oxide semiconductor layer formed of iron-indium-zinc oxide (FIZO) and a second oxide semiconductor layer formed of indium-gallium-zinc oxide (IGZO), thus being capable of exhibiting effects, such as high reliability and high electron mobility.