Ammonia synthesis catalysts with high activity under mild reaction conditions for use in synthetic chemical processes powered by renewable energy are needed. Here, we investigated the physicochemical properties and ammonia synthesis activity of lanthanoid oxide-supported Ru catalysts. The basicity of the Ru/lanthanoid oxide catalysts, as estimated by CO2 temperature-programmed desorption, was higher than that of Ru/MgO, a representative catalyst with high ammonia synthesis activity, and a positive relationship was observed between basicity and turnover frequency. The turnover frequency of Ru/Pr2O3 and Ru/La2O3 (both 0.24 s(-1)) was eight times that of Ru/MgO (0.03 s(-1)). A kinetic analysis revealed that hydrogen poisoning, which is a typical drawback of oxide-supported Ru catalysts, was markedly retarded in the Ru/lanthanoid oxide catalysts. As a result, the ammonia synthesis rate of the Ru/Pr2O3 catalyst increased with increasing reaction pressure, reaching 49 mmol g(-1) h(-1) at 400 degrees C and 1.0 MPa, which was 12 times that of Ru/MgO under the same conditions.