NdON, HfON, and their mixtures (NdxHf(1−x)ON) with different Hf contents are adopted as the gate dielectrics of pentacene organic thin‐film transistors (OTFTs). Their capacitance–voltage characteristics reveal that the Hf incorporation generates negative fixed charges in the dielectric for assisting the applied gate voltage, thus achieving a low threshold voltage for the OTFT. X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) results indicate that Hf can suppress the hygroscopicity of Nd oxide while Nd can passivate the carrier traps (e.g., oxygen vacancies) of Hf oxide, both of which can contribute to a smoother dielectric surface for the growth of larger pentacene grains on the dielectric surface. As a result, among the samples with different Hf contents, the OTFT with Nd0.85Hf0.15ON can achieve the highest carrier mobility of 1.95 cm2 V−1 s−1 due to least Coulomb scattering (associated with lowest interface‐trap density of 5.9 × 1012 cm−2 eV−1 and smallest hysteresis of 0.26 V) and least grain‐boundary scattering (associated with largest pentacene grains).