Copper nanoparticles (CuNPs) possess strong localized surface plasmon resonance (LSPR) in visible light. However, CuNPs are not chemically stable in air, which has seriously hindered the applications based on the LSPR of CuNPs. We developed an artificial method to passivate CuNPs as Al naturally does in air, preventing the oxidation of CuNPs through swift oxidation of the surface atoms via oxygen plasma irradiation. A hemispheric core-shell structure of CuNPs uniformly covered by a dense CuO shell (CuNPs@d-CuO) was constructed. The 4 nm d-CuO shell can prevent CuNPs from further oxidation. As a result, the LSPR of the CuNPs is stable in air over 180 days.