The present invention relates to a composition for an insulator of a thin film transistor, an insulator and an organic thin film transistor comprising the same. The insulator of a thin film transistor prepared with the composition of the present invention displays an excellent permittivity along with a low surface energy, and the organic thin film transistor comprising the same displays an improved organic semiconductor morphology formed on the top surface of the insulator, so that it can bring the effect of reducing leakage current density, improving charge carrier mobility, and improving current on/off ratio.