In order to modify the cycle stability of low-Co AB(5)-type alloy, a trace of boron was added in MmNi(3.8)Co(0.4)Mn(0.6)Al(0.2) hydrogen storage alloy. The low-Co AB(5)-type alloys MmNi(3.8)Co(0.4)Mn(0.6)Al(0.2)B(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 38 m/s from 310 to 566 cycles. (C) 2003 Elsevier B.V. All rights reserved.