Cobalt carbide (Co 2 C) nanoparticles were synthesized simply in liquid phase using Co(II) acetylacetonate and oleylamine under reflux conditions. Even in the presence of carbon black XC-72, Co 2 C nanoparticles were formed and simultaneously deposited on the carbon surface. The thin layer consisting of cobalt (oxy)hydroxide was present on the outermost surface of the thus-prepared Co 2 C nanoparticles, which was confirmed by X-ray photoelectron spectroscopy, but the redox treatment using a cyclic voltammetry technique gradually removed such byproduct phase. The resulting Co 2 C nanoparticles supported on XC-72 exhibited high durability for the oxygen reduction reaction.