Disclosed herein is a method for fabricating an organic thin film transistor that includes a gate electrode, a gate insulating film, source/drain electrodes and an organic semiconductor layer formed in this order on a substrate wherein the surface of the gate insulating film on which source/drain electrodes are formed is impregnated with an inorganic or organic acid, followed by annealing. According to the method, the surface of a gate insulating film damaged by a photoresist process can be effectively recovered. In addition, organic thin film transistors having high charge carrier mobility and high on/off current ratio can be fabricated.