A method of manufacturing a thin film transistor is provided. The method includes forming a lower organic semiconductor layer, forming an upper organic semiconductor layer on the lower organic semiconductor layer, the upper organic semiconductor layer having solubility and conductivity higher than those of the lower organic semiconductor layer, forming a source electrode and a drain electrode spaced apart from each other and respectively overlapping the upper organic semiconductor layer, and dissolving the upper organic semiconductor layer selectively by using the source electrode and the drain electrode as a mask.