Pentacene organic thin-film transistors (OTFTs) using LaxNb(1–x)Oy as gate dielectric with ${x}= \mathsf {1}$ , 0.946, 0.850, and 0.648 have been fabricated on vacuum tape at a temperature of 200 °C. Among them, the OTFT with La0.850Nb0.150Oy as gate dielectric has the highest carrier mobility of 4.63 cm $^{\mathsf {2}}\text{V}^{\mathsf {-1}}\text{s}^{\mathsf {-1}}$ , negligible hysteresis of −0.032 V, small subthreshold swing of 0.174 V/decade, and small threshold voltage of −1.90 V. AFM and X-ray photoelectron spectroscopy reveal that Nb incorporation can alleviate the hygroscopicity of La oxide, resulting in a smoother dielectric surface. Pentacene grain size first increases with Nb content due to smoother dielectric surface, and then decreases due to Nb-induced traps acting as nucleation sites for the growth of pentacene grains on the dielectric surface. Therefore, there exists an optimal Nb content in LaxNb(1–x)Oy that can result in the largest grain size and, hence, the highest carrier mobility.