A method of fabricating a bottom gate type organic thin film transistor is provided. The method includes the acts of: forming a gate conductive layer pattern on a substrate; forming a gate insulating layer on an exposed portion of the surface of the substrate and the gate conductive layer pattern; forming source/drain electrodes on the gate insulating layer to expose a portion of the surface of the gate insulating layer above on the gate conductive layer pattern; forming an organic semiconductor thin film on the exposed portion of the gate insulating layer; forming on the organic semiconductor thin film a passivation layer pattern exposing a portion of the surface of the organic semiconductor thin film; and forming an organic semiconductor thin film pattern by etching the exposed surface of the organic semiconductor thin film using the passivation layer pattern as an etch mask.