This paper demonstrates effects of a surface modification of polymeric gate insulators on a performance of organic thin-film transistor (CTFT). Pentacene OTFTs were fabricated with three types of polymer gate insulators-poly(4-vinylphenol) (PVP, G1) with comparably high dielectric constant, polyimide (P1, G2) with n-octadecyl (C(18)) side chain, which resulted in hydrophobicity and low dielectric constant, and surface modified PVP(G3). The G3 Was prepared by a spin-coating the solution of G2 onto the G1 film. We found that the n-octadecyl group of the G3 protruded from the surface and made the PVP Surface more hydrophobic. The less polar surface strongly improved the device performance. Subthreshold slope (s.s.) of the OTFT with G3 as the gate insulator decreased significantly to 2.7 V/dec, which was much smaller than that of OTFTs fabricated with G1 (4.0 V/dec). That is, thin layer with fewer C(18) group in contact with pentacene induced a good electrical property like lower s.s. Further the higher dielectric constant of the underlying layer resulted in higher mobility of the device. The mobility (0.50 cm(2) V(-1) s(-1)) of the OTFT with G3 as the gate insulator showed a higher value compared to that (0.25 cm(2) V(-1) s(-1)) of the OTFT with G2. (C) 2008 Elsevier B.V. All rights reserved.