The present invention relates to a method for fabricating an organic thin film transistor, including: (A) providing a gate electrode; (B) forming a gate insulating layer on the gate electrode; and (C) forming an organic active layer, a source electrode and a drain electrode over the gate insulating layer, and increasing crystallinity of the organic active layer by irradiating the organic active layer. Accordingly, through irradiation, the present invention can efficiently enhance the field effect mobility, and thereby significantly improves the device performance of an organic thin film transistor. Additionally, irradiation mentioned in the present invention also can be used for repairing an organic thin film transistor.