A method of manufacturing an organic semiconductor device is provided. The method includes providing a substrate, forming a sacrificial layer on the substrate, forming a patterned organic semiconductor layer on the sacrificial layer, forming an insulating layer on the patterned organic semiconductor layer, forming a gate electrode on the insulating layer, separating the sacrificial layer and the substrate from the patterned organic semiconductor layer, and forming a source/drain electrode on the patterned organic semiconductor layer, so as to provide a simple and effective method of manufacturing the organic semiconductor device.