It has been challenging to synthesize palladium (Pd) and ruthenium (Ru) alloy nanoparticles of solid solution in atomic scale, thus preventing us to study the size effect on the catalytic activity. Here we reported the synthesis of PdRu alloy nanoparticles through polyol reduction method, with the size from 6.0 to 16.6 nm. Elemental mapping indicated that Pd and Ru with 1:1 ratio are evenly distributed over the alloy nanoparticles. XRD of PdRu alloy nanoparticles displayed the characteristic diffraction patterns of face centered cubic (fcc) of Pd and invariable inter plane distance of (1 1 1). The band width of valence state increased from 5.7 to 6.2 eV when the diameter of the nanoparticles increased from 6.0 to 16.6 nm. The size effect of PdRu alloys towards electrooxidation of formic acid was examined on the support of CNT. PdRu alloy nanoparticles with diameter of 15.6 nm performed best among all the samples, 5.7 times higher than that of commercial Pd/C with formic acid. PdRu alloy nanoparticles also demonstrated optimum catalytic activity towards methanol oxidation in alkaline medium solution and the best performance was achieved by PdRu alloy nanoparticles with diameter of 15.6 nm, which was 2.5 times that of Pd/C. A volcano shape dependence of catalytic activity towards both formic acid and methanol oxidation on the size of PdRu alloy nanoparticles was observed. This work provided a vial strategy for the size controlled synthesis of alloy nanoparticles of solid solution in atomic scale, especially for those elements that are immiscible, unraveled the size effect on the electronic structure and catalytic activity on metal nanoparticles and may shed light on the optimization of alloy noble metal nanoparticles. (C) 2018 Elsevier Inc. All rights reserved.