High field-effect mobility organic thin-film transistors with nanoparticle polymer composite/polymer bilayer insulators were fabricated. A device composed of a polyvinylphenol (PVP) insulator filled with barium strontium titanate (BST) nanoparticles and coated with a thin polystyrene film was found to exhibit a field-effect mobility (1.2 cm2 V-1 s-1 for a pentacene device) and a field-induced current that are superior to those of devices with only a PVP insulator or a BST-filled PVP insulator. These improvements in the performance are due to an increase in the capacitance and a reduction in the interface roughness of the insulator.