A series of core/shell CdSe/CdS nanocrystals (NCs) with different shell thicknesses were prepared, and the electrochemical band gap (E-g(CV)), which was characterized with the cyclic voltammogram, was increased with the growth of CdS shell whereas the optical band gap (E-g(OP)), which was gotten by optical absorption/photoluminescence, was decreased with the increase of the shell thickness. For comparison, a series of CdSe NCs with different diameter were prepared also, and both the E-g(CV) and E-g(OP) values exhibit similar dependence on the NC's size. It is suggested that the Egv value is affected by not only the electronic energy level of the CdSe core but also the CdS shell surface. (C) 2015 Elsevier B.V. All rights reserved.