In order to modify the cycle stability of low-Co AB5-type alloy, a trace of boron was added in MmNi3.8Co0.4Mn0.6Al0.2 hydrogen storage alloy. The low-Co AB5-type alloys MmNi3.8Co0.4Mn0.6Al0.2B x (x=0, 0.1, 0.2, 0.3, 0.4) were prepared by cast and rapid quenching. The cycle lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of boron additive on the microstructures and cycle lives of as-cast and quenched alloys were investigated comprehensively. The obtained results showed that the addition of boron could dramatically enhance the cycle lives of the as-cast and quenched alloys. When boron content x increases from 0 to 0.4, the cycle lives of the as-cast alloys were increased from 118 to 183 cycles, and for as-quenched alloys with quenching rate of 38m/s from 310 to 566 cycles.