A method of manufacturing a thin film transistor capable of simplifying a substrate structure and a manufacturing process is disclosed. The method of manufacturing a thin film transistor array substrate includes involves a three-round mask process, which includes: forming a gate pattern on a substrate; forming a gate insulating film on the substrate having the gate pattern thereon; forming a source/drain pattern and a semiconductor pattern; forming a passivation film to protect the thin film transistor on an entire surface of the substrate; forming a photo-resist pattern on the passivation film; patterning the passivation film using the photo-resist pattern to form a passivation film pattern; and forming a transparent electrode pattern being extended from a lateral surface of the passivation film pattern and formed at an area except for the passivation film pattern.