A novel high-κorganometallic lanthanide complex, Eu(tta)3L (tta=2-thenoyltrifluoroacetonate, L = 4,5-pinene bipyridine), is used as gate insulating material to fabricate low-voltage pentacene field-effect transistors (FETs). The optimized gate insulator exhibits the excellent properties such as low leakage current density, low surface roughness, and high dielectric constant. When operated under a low voltage of 615 V, the pentacene FET devices show the attractive electrical performance, e.g. carrier mobility (μFET) of 0.17 cm2V611s611, threshold voltage (Vth) of 610.9 V, on/off current ratio of 5 × 103, and subthreshold slope (SS) of 1.0 V dec611, which is much better than that of devices obtained on conventional 300 nm SiO2substrate (0.13 cm2V611s611, 617.3 V and 3.1 V dec611forμFET,VthandSSvalue when operated at 6130 V). These results indicate that this kind of high-κorganometallic lanthanide complex becomes a promising candidate as gate insulator for low-voltage organic FETs.