Copper nanoparticles are produced by high-voltage electrical explosion of copper wires. The high-voltage breakdown experimental setup for copper is built. The morphology and composition of the breakdown material are tested by the transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and X-ray diffractometry (XRD) and energy dispersive spectroscopy(EDS) methods, respectively. Based on the morphology, size distribution, elemental spectrum (EDS), and XRD analysis of the breakdown material, the phase characteristics of high-voltage breakdown copper wire are studied. The results show that wire is fully ionized under high pressure, forming a filamentous distribution which is composed of condensation of nanoparticles. The diameters of copper nanoparticles are between 30 nm and 60 nm. The nanoparticles product is composed of Cu and O elements. The product is a mixture of metallic copper, cupric oxide and cuprous oxide. The particle size and its product composition are controlled by varying length and diameter of the copper wire, discharge voltage, etc.