Copper oxide nanoparticles with an average diameter of about 10 mm were synthesized via thermal decomposition of CuC2O4 precursor at 400 C. The nanoparticles were investigated using XRD, HRTEM, FESEM, and UV-vis spectrophotometry. The optical absorption spectrum indicates that the CuO nanoparticles have a direct band gap of 2.75 eV. The electrochemical tests show that the ultrafine CuO nanoparticles, as a promising electrode material, can deliver a large reversible discharge capacity of about 810 mA h g(-1). We suggest that if the particles size is too small, the too large specific surface area of the CuO may lead to increase the capacity of the first discharge. (c) 2007 Elsevier Ltd. All rights reserved.