In the present study, the formation of hollow Cu oxide nanoparticles through the oxidation process at temperatures from 200 to 300℃ has been studied by transmission electron microscopy with Cu nanoparticles produced by the plasma arc discharge method. The Cu nanoparticles had a thin oxide layer on the surface at room temperature and the thickness of this oxide layer increased during oxidation in atmosphere at 200-300℃ However, the oxide layer consisted of Cu2O and CuO after oxidation at 200℃ whereas this layer was comprised of only CuO after oxidation at 300℃ On the other hand, hollow Cu oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process.