Cobalt oxide (Co3O4) nanoparticles have been successfully prepared via a simple reduction-oxidation route at room temperature and normal pressure, using cobalt chloride as the reactant and sodium acetate as the basic agent. Owing to employing a gamma -irradiation technique, no extra reducing reagent was introduced into the system. The experiment contained two steps: first, cobalt (II) ions were reduced to cobalt atoms by gamma -irradiation in nitrogen. Then, cobalt atoms were oxidized to cobalt oxide (Co3O4) in air. The TEM images showed that cobalt oxide powders consisted of crystallized spherical particles and amorphous wire-like particles.