In order to improve the cycling 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 -x Co0.45Al (x) (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were prepared by casting and rapid quenching. The effects of the substitution of Al for Ni on the structure and electrochemical performance of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the substitution of Al for Ni has an inappreciable influence on the abundance of the LaNi2 phase in the as-quenched alloy, while it increases the amount of the LaNi2 phase in the as-cast alloys. In addition, the substitution of Al for Ni is unfavourable for the formation of an amorphous in the as-quenched alloy. The results obtained by the electrochemical measurement indicate that the cycling stabilities of the as-cast and quenched alloys are significantly ameliorated with increasing Al content. When Al content increases from 0 to 0.4, the cycle life of the as-cast and quenched (30 m/s) alloys enhances from 72 to 132 cycles and from 100 to 136 cycles, respectively.