Provided are a thin film transistor (TFT) including a selectively crystallized channel layer, and a method of manufacturing the TFT. The TFT includes a gate, the channel layer, a source, and a drain. The channel layer is formed of an oxide semiconductor, and at least a portion of the channel layer contacting the source and the drain is crystallized. In the method of manufacturing the TFT, the channel layer is formed of an oxide semiconductor, and a metal component is injected into the channel layer so as to crystallize at least a portion of the channel layer contacting the source and the drain. The metal component can be injected into the channel layer by depositing and heat-treating a metal layer or by ion-implantation.