Oleic acid-capped titanium dioxide (OA-TiO2) nanoparticles were solution-processed to form homogeneous dielectrics for organic thin-film transistors (TFTs) of top-gate and bottom-gate configurations. The OA-TiO2 nanoparticles were well-dispersed into the organic solvent and spin-coated to give homogeneous films. These nanoparticle films showed a dielectric constant of about 5.3 and low leakage current density of ∼3×10-8 A/cm2 under an electric field of 1 MV/cm. Poly(3,3‴-didodecylquaterthiophene) and pentacene TFTs with OA-TiO2 dielectrics exhibited mobilities of 0.05±0.02 and 0.2±0.05 cm2 V-1 s-1, respectively, with on/off ratios of 103–105. This material appears to be useful for applications in printable organic TFTs.