Low-voltage pentacene-based organic thin-film transistors (OTFTs) have been fabricated using the high-K organo-metallic lanthanide complex, Tb(tta)(3)L-2NR (tta = 2-thenoyltrifluoroacetonate, L-2NR = (-)-4, 5-pinene bipyridine) as the gate dielectric material. The optimized gate insulator exhibits a low leakage current density of <10-7 A cm(-2) under bias voltage of -5 V, a smooth surface with RMS of about 0.40 nm, a high capacitance of 43 nF cm(-2) and an equivalent K value of 7. The obtained OTFTs show high electric performance with carrier mobility of 0.20 cm(2) V-1 s(-1), on/off ratio of 4 x 10(5), threshold voltage of -0.6 V, and subthreshold slope of 0.7 V dec(-1) when operated at -5 V. The results demonstrate the organometallic lanthanide complex is a promising candidate as gate insulator for low-voltage OTFTs. (C) 2017 Elsevier B.V. All rights reserved.