Nanoparticles of Pd-Sn were prepared under various conditions by applying ultrasonic irradiation. The Pd-Sn nanoparticles thus prepared were about 3-5 nm in diameter. The Pd in Pd-Sn nanoparticles was found to be mostly in the metallic state. The PdSn nanoparticles have a better oxygen reduction reaction (ORR) activity than Pt and Pd nanoparticles in alkaline media. The mole ratio of Pd to Sn metal ions, their initial concentrations, the amount of ethanol, which served as the reducing agent for metal ions, and the amount of citric acid in the synthesizing solution were found to affect the size and distribution of obtained Pd-Sn nanoparticles. Therefore, the electrocatalytic activity for oxygen reduction was controlled by the above factors. Particularly, the amount of citric acid is important to control the surface modification on Pd-Sn particles to adjust the electrocatalytic activity for oxygen reduction.