Nanoscale COO particles were synthesized by the decomposition of CoCO3 nanorods, which obtained by a precipitation method. Two groups of positive electrodes for nickel-metal hydride (Ni/MH) batteries were prepared, respectively, with as-prepared nanoscale CoO and commercial submicron CoO as conducting agent. The electrochemical properties of the electrodes were measured by cycling the cells in a three-compartment system. By substituting the submicron CoO with nanoscale CoO, the reversibility and reversible potential of the electrochemical reaction were both enhanced, and the formation of gamma-NiOOH during the charge-discharge process was restricted to a low level. Accordingly the positive electrodes with nanoscale CoO exhibited increased specific capacity and discharge voltage and were easy to be activated as compared to those with submicron CoO. The results indicate that the nanoscale CoO is better as additive for the positive electrodes of the Ni/MH rechargeable batteries than the common submicron CoO. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.