In order to modify its cycle stability, a trace amount of boron was added in low-Co AB5-type hydrogen storage alloy. The low-Co AB5-type MmNi3.8Co0.4Mn0.6Al0.2B(x=0, 0.1, 0.2, 0.3, 0.4)hydrogen storage alloys were prepared by cast and rapid quenching. The x microstructures and electrochemical performances of the as-cast and quenched alloys were analyzed and measured. The effect of boron additive on the microstructures and cycle lives of the alloys were investigated comprehensively. The experimental results show that the addition of boron dramatically enhances the cycle lives of the as-cast and quenched low-Co AB5-type hydrogen storage alloys, whereas the action mechanism of boron is completely different.