A method manufacturing a thin film transistor is provided. A gate, a first insulation layer covering the gate, a semiconductor layer over the gate, and a first photoresist pattern are sequentially formed on a substrate. The semiconductor layer is patterned into a channel layer by using the first photoresist pattern as a mask and the first photoresist pattern is subsequently shrunken to remain a portion of the first photoresist pattern on the channel layer. A conductive material layer covering the remained portion of the first photoresist pattern, the channel layer and the first insulation layer is patterned by using a second photoresist pattern as a mask to form a source and a drain separated by a gap region exposing the remained portion. The second photoresist pattern and the remained portion are removed by performing a stripping process to expose the channel layer between the source and the drain.