Ru/CeO2-PrOx catalysts for NH3 synthesis were prepared using the urea-based homogeneous co-precipitation method, and the effect of Pr-doped CeO2 support on the catalytic activity was investigated. An optimal range of Pr content (25–75 %) improved NH3 synthesis activity. CO pulse adsorption and operando X-ray absorption fine structure (XAFS) measurements revealed that the increase in NH3 synthesis activity was due to the improvement in Ru dispersion by the interaction between PrOx and Ru, rather than the generally discussed Ce3+/Ce4+ ratio. However, excess Pr doping prevented the Ru reduction and formation of active sites. In this Ru/CeO2-PrOx catalyst system, the activity is governed by the number of active sites that can be controlled by the Pr content. In the high-pressure evaluation (5 MPaG), the NH3 synthesis rate of the Ru/CeO2-PrOx catalyst (Ce/Pr = 1) was 9.0 mmol g-1h−1, which is 1.4 times higher than that of the Ru/CeO2 catalyst.