Nanoscale cobalt oxide (CoO) particles were synthesized by analysis of CoCO3 in vacuum. Four groups of sealed Ni/MH batteries with different ratio of nanoscale CoO in the positive electrodes were assembled. The overall characteristics of Ni/MH batteries were investigated at different discharge rates at room temperature. The high-rate discharge performance of the Ni/MH batteries was improved by the addition of nanoscale CoO in positive electrode as compared with the addition of normal CoO. Under high-rate discharge conditions, the batteries with sufficient nanoscale CoO in positive electrodes presented much better cycling stability, higher discharge mean voltage, lower internal resistance and higher high-rate capacity. The addition of 8wt.% nanoscale CoO was proved a desired amount to modify the battery performance at high discharge rates. Too much nanoscale CoO contributed no effect on the improvement of overall performance of Ni/MH batteries.