A method of manufacturing a thin film transistor that provides high electric field mobility is disclosed. In one embodiment, the method includes: a) forming an amorphous silicon layer and a blocking layer on an insulating substrate; b) forming a photoresist layer having first and second photoresist patterns on the blocking layer, the first and second photoresist patterns spaced apart from each other; c) etching the blocking layer using the first photoresist pattern as a mask to form first and second blocking patterns; d) reflowing the photoresist layer, so that the first and second photoresist patterns abut on each other to entirely cover the first and second blocking patterns; e) forming a capping layer and a metal layer over an entire first surface of the insulating substrate; f) removing the photoresist layer to expose the blocking layer and an offset region between the blocking layer and the metal layer; g) crystallizing the amorphous silicon layer by diffusing metals in the metal layer through the capping into the amorphous silicon layer; h) etching the poly silicon layer using the first and second blocking patterns as a mask to form first and second semiconductor layers; and i) removing the first and second blocking patterns.