Large-scale cupric oxide (CuO) nanowires have been synthesized by thermal evaporating copper foils in O 2 ambient at the temperatures from 300°C to 900°C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were performed on the study of the CuO nanowires. XRD studies reveal the possibility of two-steps of nanowire formation; HRTEM image shows the bicrystal structure of the nanowire; SEM studies indicate the relationship between the morphology of the prepared nanowires and the growth conditions. The growth of the nanowires seems to be vapor–solid (VS) mechanism.