In this paper, we report the sphere-like C/CuO and CuO nanocrystals that have been fast prepared by the microwave-assisted hydrothermal method in the presence of green additive D-glucose. The rapid formation mechanism of the C/CuO and CuO nanocrystals and how the D-glucose worked in the microwave-assisted hydrothermal system were discussed in details. The crystal phase, morphology and electrochemical performance were characterized by means of X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and electrochemical testing. The XRD patterns indicated that the CuO nanocrystals are pure with monoclinic structure and TEM images showed that the size of the samples could be tunable by adjusting the ratio of h-glucose and metal salt. Finally, a possible growth mechanism for the C/CuO spheres and CuO nanocrystals was proposed and discussed. The electrochemical performance of the products may be influenced by the size and uniformity of the nanocrystals. (C) 2013 Elseviei B.V. All rights reserved.