Ag nanoparticles (AgNPs) were prepared by a galvanic reaction between Ag+ ions and Co metal particles supported on carbon. Variation of the preparation conditions provided a series of AgNPs with various sizes (ca. 2-12 nm). By using the various AgNPs, the size effect on the oxygen reduction reaction activity was investigated. The mass activity of AgNPs increased with a decrease of the size. On the other hand, the Ag-surface-area-specific activity decreased with a decrease of the size. [GRAPHICS] .