Provided are a composition for a photo-reactive organic polymeric gate insulating layer and an organic thin film transistor using the same. The composition for the photo-reactive organic polymeric gate insulating layer comprises poly (vinyl) phenol and a photo-reactive material, and the organic thin film transistor has a photo-reactive organic polymeric gate insulating layer formed of the composition. The composition for the photo-reactive organic polymeric gate insulating layer can add the photo patterning characteristics to an organic polymer and can form the layer with the enhanced electrical characteristics.