A poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA) and titanium dioxide (TiO2) composite was fabricated to use as a gate insulator in pentacene-based organic thin-film transistors (OTFTs). The dispersion stability was confirmed by observing the sedimentation time of TiO2 nanoparticles in the PMMA-co-MAA solution, which is essential to avoid a severe gate-leakage current in OTFTs. From the measured capacitance-frequency characteristics, a dielectric constant value of 4.5 was obtained for the composite film and 33 for the PMMA-co-MAA film. Consequently, we could enhance the field-induced current and reduce the threshold voltage of OTFT by adopting the composite insulator, without augmenting the gate-leakage current. (C) 2009 Elsevier B.V. All rights reserved.