A series of solution-processable oleic-acid capped barium titanate and strontium titanate nanoparticles was synthesized and spin-coated to form homogeneous high-k dielectric films for organic thin-film transistors (TFTs). The dielectric constant k of the nanoparticle films was tunable in the range from 4.1 to 9.3 by altering the molar ratio of oleic-acid surfactant to synthesis precursor. Low-voltage modulated high-performance organic TFTs were fabricated using nanoparticle films as the dielectric components. Flexible bottom-gate pentacene TFTs exhibited outstanding device performance with field-effect mobility, μ, in the range of 2.0−3.5 cm2 V−1 s−1 and on/off ratios of about 1 × 104 at low gate voltage. Top-gate poly(3,3′′′-didodecylquaterthiophene) TFTs also showed high device performance with μ of 0.05−0.1 cm2 V−1 s−1 and on/off ratios of 1 × 103 to 1 × 104. The low-voltage performance of the TFTs could be attributed to a low density of trapped states at the interfaces between the organic semiconductors and the nanoparticle dielectric films. This research provides a series of promising dielectric materials for fabrication of superior organic TFTs through a solution process and fundamentally suggests that low trapped state density at the semiconductor/dielectrics interface may be an important factor to achieve low-voltage modulation in organic TFTs.