In order to improve the cycle stability of La-Mg-Ni system (PuNi3-type) hydrogen storage alloy, Ni in the alloy was partially substituted by Al, and La0.7Mg0.3Ni2.55-xCo0.45Alx (x=0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of substituting Ni with Al on the microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni-3, the LaNi5 and the LaNi2 phases. The substitution of Al for Ni increases the amount of the LaNi2 phase in the as-cast alloys, but it has an inappreciable influence on the abundance of the LaNi2 phase in the as-quenched alloy. The results obtained by SEM and TEM indicate that the substitution of Al for Ni leads to the grains of the as-quenched alloys coarsen and restrain an amorphous phase formed in the as-quenched alloys. The results of the electrochemical measurement indicate that the capacities of the as-cast and quenched alloys monotonously decrease and their cycle stabilities significantly increase with the increase of Al content. When the amount of substitution Al for Ni increased from 0 to 0.4, the cycle life of the as-cast alloys increased from 72 to 132 cycles, and that of the as-quenched (20 m s(-1)) alloys from 86 to 134 cycles. (c) 2007 Elsevier B.V. All rights reserved.