The La-Mg-Ni-based PuNi3-type La0.7Mg0.3Ni2.55-xCo0.45Cux (x=0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. Ni in the alloy was partly substituted by Cu in order to improve the cycle stabilities of the alloys. The effects of the substitution of Cu for Ni on the microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the as-cast and quenched alloys have a multiphase structure which is composed of two main phases (La, Mg)Ni-3 and LaNi5 as well as a residual phase LaNi2. The substitution of Cu for Ni leads to an obvious increase of the LaNi2 phase in the as-cast alloys, and it also helps the formation of a desired amorphous phase in the as-quenched alloy. The results of the electrochemical measurement indicate that the substitution of Cu for Ni obviously decreases the discharge capacities of the as-cast and quenched alloys, but it significantly improves their cycle stabilities. The substitution of Cu for Ni slightly improves the discharge potential characteristics and the activation capabilities of the as-cast alloys, but it leads to reversal results for the as-quenched alloys. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.