Provided is an insulating layer in which an inorganic material is added to an organic polymer to thereby improve the insulating properties, an organic thin film transistor using the insulating layer, and a method of fabricating the organic thin film transistor. An insulating layer for an organic thin film transistor including a vinyl polymer and an inorganic material is provided. Here, a weight ratio of the vinyl polymer to the inorganic material may be in the range of 1:0.0001 to 1:0.5. Accordingly, it is possible to fabricate a thin film at low temperature and, further, to fabricate an insulating layer having a high-dielectric constant, not affecting other layers formed in the previous processes during the formation of the insulating layer.