Developing catalyst of high performance and low cost toward the electro-oxidation of formic acid on the anode of fuel cell is critical for the commercialization of direct formic acid fuel cells. Here we reported the synthesis of PdxRu10-x (x = 1,3,5,7,9) nanoparticles (NPs) by concurrent reduction of Pd2+ and Ru2+ in polyol solution at 200 degrees C. The particle size of the obtained NPs was confined at 5-15 nm in diameter. X-ray diffraction (XRD) analysis revealed face-centered cubic (fcc) crystal structure for PdxRu10-x (x = 3,5,7,9), with the lattice parameter proportional to the Pd content. The formation of the solid solution in atomic scale was confirmed for the alloy nanoparticles by XRD and the elemental mapping. Williamson-Hall method revealed that the stacking fault was dependent on the alloying extent of the alloy nanoparticles and reached the minimum for Pd5Ru5, which exhibited the highest activity towards formic acid oxidation among all these prepared samples, with mass activity of 12.6 times higher than that of commercial Pd/C. It was observed that the highest catalytic activity was in agreement with the minimum of the stacking fault of the alloy nanoparticles. (C) 2017 Elsevier Ltd. All rights reserved.