The structural characteristics of monoclinic CuO nanowires (NWs) fabricated by heating pure Cu in ambient air were investigated by electron microscopy. Besides the single-crystalline NW, four different twinned NWs with twinning planes of (11\bar 1), (002), (110) and {{(20\bar 2)}} have been found. The twin boundaries are generally in parallel with the NW axial direction. To the best of the authors' knowledge, the {{(11\bar 1)}} and (110) twins are reported for the first time in CuO. Moreover, the prevailing existence of {{(11\bar 1)}} and (002) twinned NWs could be closely related to the NW growth as well as the oxidation processes of Cu. The presented results provide a systematic investigation on the twin structures of CuO NWs, which may open up a pathway to explore new potential applications of CuO nanostructures.