An organic thin film transistor (OTFT) comprising a gate electrode, a gate insulating film, an organic active layer and a source/drain electrode, or a gate electrode, a gate insulating film, a source/drain electrode and an organic active layer, sequentially formed on a substrate, wherein the gate insulating film is a multi-layered insulator comprising a first layer of a high dielectric material and a second layer of an insulating organic polymer compatible with the organic active layer, the second layer being positioned directly under the organic active layer. The OTFT of the present invention shows low threshold and driving voltages, high charge mobility, and high I_on/I_off, and it can be prepared by a wet process.