The present invention relates to an organic thin film transistor comprising a photocurable transparent inorganic/polymer composite layer as a gate insulator layer in which metal oxide nanoparticles are generated within a photocurable transparent polymer through sol-gel and photocuring reactions and whose permittivity is easily regulated; and a fabrication method thereof. Since the organic thin film transistor according to the present invention utilizes the photocurable transparent inorganic/polymer composite layer showing a significantly high and readily controllable permittivity as a gate insulator, it is capable of operating under low voltage conditions and has a high on/off current ratio due to low leakage current. Further, the organic thin film transistor according to the present invention preserves the unique properties of the photocurable transparent polymer, enabling the formation of a photocurable micropattern of a gate insulator having high processibility.