Effect of La/Mg ratio on the structure and electrochemical properties of La x Mg3−x Ni9 (x=1.6–2.2) ternary alloys was investigated. All alloys are consisted of a main phase with hexagonal PuNi3-type structure and a few impurity phases (mainly LaNi5 and MgNi2). The increase of La/Mg ratio in the alloys leads to an increase in both the cell volume and the hydride stability. The discharge capacity of the alloys at 100mA/g increases with the increase of La/Mg ratio and passes though a maximum of 397.5mAh/g at x=2.0. As the La/Mg ratio increases, the high-rate dischargeability of the alloy electrodes at 1200mA/g HRD1200 decreases from 66.7% (x=1.6) to 26.5% (x=2.2). The slower decrease of HRD1200 (from 66.7 to 52.7%) of the alloys with x=1.6–2.0 is mainly attributed to the decrease of electrocatalytic activity of the alloys for charge-transfer reaction, the more rapid decrease of HRD1200 of the alloys with x>2.0 is mainly attributed to the lowering of the hydrogen diffusion rate in the bulk of alloy. The cycling capacity degradation of the alloys is rather fast for practical application due to the corrosion of La and Mg and the large V H in the hydride phase.