The effect of La/Ce ratio on the structure and electrochemical characteristics of the La0.7−x Ce x Mg0.3Ni2.8Co0.5 (x =0.1, 0.2, 0.3, 0.4, 0.5) alloys has been studied systematically. The result of the Rietveld analyses shows that, except for small amount of impurity phases including LaNi and LaNi2, all these alloys mainly consist of two phases: the La(La, Mg)2Ni9 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonal CaCu5-type structure. The abundance of the La(La, Mg)2Ni9 phase decreases with increasing cerium content whereas the LaNi5 phase increases with increasing Ce content, moreover, both the a and cell volumes of the two phases decrease with the increase of Ce content. The maximum discharge capacity decreases from 367.5mAhg−1 (x =0.1) to 68.3mAhg−1 (x =0.5) but the cycling life gradually improve. As the discharge current density is 1200mAg−1, the HRD increases from 55.4% (x =0.1) to 67.5% (x =0.3) and then decreases to 52.1% (x =0.5). The cell volume reduction with increasing x is detrimental to hydrogen diffusion D and accordingly decreases the low temperature dischargeability of the La0.7−x Ce x Mg0.3Ni2.8Co0.5 (x =0.1–0.5) alloy electrodes.