For decreasing the cost of AB_5-type hydrogen storage alloys, a low-cobalt Ml_(0.9)Mg_(0.1)Ni_(3.4)Co_(0.3)Al(0.3) alloy was investigated by examining the alloy structure, phase composition and some properties. Its properties were compared with those of a commercial alloy MmNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3). The experiments' outcome shows that the low-cobalt alloy is composed of Mg-free LaNi_5 phase as matrix and Mg-contained LaNi3 phase as secondary phase. The hydrogen storage capacity (1.36wt% and 1.37wt% respectively), the greatest discharge capacity (320 mAh/g and 324 mAh/g respectively) and cycling stability (the residual capacity of two alloys is all 88% ) of Ml_(0.9)Mg_(3.4)Ni_(3.4)Co(0.3)Al_(0.3) are as good as those of MmNi_(3.55)Co(0.75)Mn(0.4)Al_(0.3). However, after the discharge current density exceeds 300 mA/g, the high-rate discharge abilities (HRD) of Ml_(0.9)Mg_(0.1)Ni_(3.4)Co_(0.3)Al_(0.3) are apparently better than those of MmNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3).It is because there exists Mg-contained LaNi_3 phase as secondary phase which improves the electrochemical catalytic activity of alloy particles surface and the structure toughness of alloy particles.