We propose a way to fabricate polymer insulators with a high gate capacitance for low-operating voltage organic field-effect transistors (OFETs). The insulator consists of spin-coated cyanoethylpullulan as a high-k polymer and chemical vapor deposited parylene-C as a covering layer. Parylene-C layer is insoluble in a common organic solvent, so the dielectric system can be fabricated on a layer of solution processable organic semiconductors such as poly( 9,9-dioctylfluorenecobithiophene) (F8T2). The OFET shows a field- effect mobility of 3.4 x 10(-3) cm(2)/Vs, a threshold voltage of -1V, and an on/off current ratio of 5.9 x 10(2). We successfully observed low-voltage operation in OFETs with this dielectric system.