Non-stoichiometric La-rich MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3)B(0.3) hydrogen storage alloys using B-Ni or B-Fe alloy as additive and Ce-rich MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3)B(0.4) one using pure B as additive have been prepared and their microstructure, thermodynamic, and electrochemical characteristics have been examined. It is found that all investigated alloys show good activation performance and high-rate dischargeability though there is a certain decrease in electrochemical capacities compared with the commercial MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3) alloy. MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3)B(0.3) alloys using B-Ni alloy as additive or adopting Ce-rich mischmetal show excellent rate capability and can discharge capacity over 190 mAh/g even under 3000 mA/g current density, which display their promising use in the high-power type Ni/MH battery. The electrochemical performances of these MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3)B(0.3) alloys are well correlated with their microstructure, thermodynamic, and kinetic characteristics.