An organic thin film transistor is disclosed, which comprises an azole-metal complex compound used as the gate insulating layer. The method of making the self-assembled gate insulating layer is a water-based processing method that enables the azole-metal complex compound to be self-formed on the patterned gate electrode in a water-based solution and serves as a gate insulating layer. The organic thin film transistor (OTFT) of the present invention comprises the azole-metal complex compound used in the gate insulating layer, therefore can be manufactured in a simple, quick, easy way for large quantities, and low cost.