Provided are a composition for thermosetting organic polymeric gate insulating layer and an organic thin film transistor using the same. The composition for thermosetting organic polymeric gate insulating layer contains a thermosetting material in polyvinyl phenol as an organic polymeric gate insulating layer material, to improve a chemical resistance and an insulating property, and an organic thin film transistor includes a thermosetting organic polymeric gate insulating layer formed of the composition. The composition for thermosetting organic polymeric gate insulating layer endows an organic polymer with a thermosetting property, thereby being capable of improving the chemical resistance and the insulating property as well as of forming a layer having a device property improved.