Metal nanoparticles, namely Co, Co5Fe2,Co5Pd1 and Co5Fe2Pd1, were successfully synthesized by applying an ultrasonic irradiation method, and their electro-catalytic activities toward ORR were also evaluated in 0.1 M KOH solution. Co5Fe2Pd1 nanoparticles with an average diameter of 3 to 5 nm were found mostly in the metallic state. Due to the trimetallic microalloying effect, Co5Fe2Pd1 nanoparticles exhibited higher ORR activity than those of Co, Co5Fe2 and Co5Pd1 nanoparticles, even higher than that of commercial Pt/C catalyst. Also, Co5Fe2Pd1 nanoparticles were more tolerant than commercial Pt/C catalyst to the poisoning effect of methanol, whereas on them the ORR proceeded predominately via the four-electron pathway. The concentration of citric acid was also found to affect the size distribution of the Co5Fe2Pd1 nanoparticles, and thus influenced their electro-catalytic activities for ORR.