Chemical stability of metal nanowires is a great concern for many practical applications. Metal nanowires deposited in the pores of various templates have been considered oxidization-resistant because the templates provide natural protection. Here we present the first ever attempt to investigate the oxidation behavior of copper nanowires supported in a template matrix using a surface-plasmon-based optical spectroscopy method combined with high-resolution transmission electron microscopy (TEM). Although the nanowires are protected by being embedded in the polymer template matrix, unexpected oxidation is clearly observed in TEM imaging, which unambiguously reveals the possible underlying oxidation mechanism responsible for the oxidation passivation. By measuring the optical properties with localized surface plasmon resonance spectroscopy, the oxidation behavior of the copper nanowires is studied as a function of storage time in both air and vacuum and thus uncovers the oxidation dynamics of copper. Finally, we demonstrate that the oxidation can be entirely prevented by sealing the open ends of the wires, which is confirmed by monitoring the electrical resistance of single nanowires. Our results have significant importance in understanding the oxidation behavior of metal nanostructures in general and also provide useful guidelines to estimate their electrical functionality in optoelectronic devices.