An organic thin film transistor that can prevent damage to source and drain electrodes when patterning an organic semiconductor layer, and a method of manufacturing an organic light emitting display device comprising the organic thin film transistor. The organic thin film transistor includes a source electrode and a drain electrode; an organic semiconductor layer that contacts the source and drain electrodes, and has an ashed surface except a channel area between the source and drain electrodes; a gate electrode insulated from the source electrode, the drain electrode, and the organic semiconductor layer; and a gate insulating film that insulates the gate electrode from the source electrode, the,drain electrode, and the organic semiconductor layer.