We introduce a photo-curable polyimide-based gate insulator for organic thin-film transistors (OTFTs) that allows low-temperature and solution-based processing and provide low leakage current density and high field-effect mobility in devices. Organic gate insulator (PI-TTE) was prepared from a blend of 75.2 wt% hydroxyl group containing polyimide (PI) and 23.8 wt% trimethylolpropane triglycidyl ether as the crosslinker, 0.5 wt% benzoyl peroxide, and 0.5 wt% triphenylsulfonium triflate as the photoacid generator (PAG). PI-TTE showed extremely low leakage current density as 2.33 x 10(-10) A/cm(2) at 3.3MV/cm and exhibited a very stable capacitance (96.74 pF/mm(2)) and it is unchangeable up to 600 hrs. Pentacene TFT using PI-TTE as a gate dielectric showed a field effect mobility as 0.203 cm(2) /Vs and an on/off ratio of 1.55 x 10(5) with almost no hysteresis.