The cyclic voltammetry indicated that the oxygen reduction reaction (ORR) proceeded by the four-electron pathway mechanism on larger Ag particles (174nm), and that the ORR proceeded by the four-electron pathway and the two-electron pathway mechanisms on finer Ag particles (4.1nm), simultaneously. The kinetics towards ORR was measured at a rotating disk electrode (RDE) with Ag/C electrode. The number of exchanged electrons for the ORR was found to be close to four on larger Ag particles (174nm) and close to three on finer Ag particles (4.1nm). The zinc-air battery with Ag/C catalysts (25.9nm) was fabricated and examined.