The structures, hydrogen storage property and electrochemical properties of the La0.7Mg0.3Ni2.55-xCo0.45Mnx (x = 0.0, 0.1, 0.2,0.3,0.4,0.5) electrode alloys has been studied systematically. It can be found that, by X-ray powder diffraction, the alloys are all consisted of the (La,Mg)Ni-3 Phase and the LaNi5 phase, and the lattice parameters and cell volumes of both the (La,Mg)Ni-3 phase and the LaNi5 phase increase with increasing Mn content in alloys. The P-C isotherms curves indicate that the hydrogen storage capacity first increases and then decreases with increasing x, and the equilibrium pressure decreases. The electrochemical measurements show that the maximum discharge capacity increases from 342.6 (x = 0.0) to 368.9 mA h/g (x = 0.3) and then decreases to 333.5 mA h/g (x = 0.5). For the discharge current density of 1000 mA/g, the high rate dischargeability (HRD) of the alloy electrodes increases from 55.8% (x = 0.0) to 72.3% (x = 0.4) and then decreases to 70.0% (x = 0.5). Moreover, according to the electrochemical impedance spectroscopy, linear polarization and anodic polarization measurements, the exchange current density I-0 and the limiting current density I-L of the alloy electrodes also all increase first and then decrease with increasing Mn content in alloys. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.