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Microstructure, hydrogen storage and electrochemical performances of Co-added La0.75Mg0.25Ni3.5?xCox (x=0, 0.2, 0.5 at%) alloys are studied. XRD and rietveld refinement results suggest that the samples are mainly composed of (LaMg)Ni3, (LaMg)2Ni7 and LaNi5 phases, Co substitution for Ni changes the phase abundance, but not the phase composition. With the rising of Co content, the amount of (LaMg)2Ni7 phase decreases, but the amount of LaNi5 phase increases, while the amount of (LaMg)Ni3 phase firstly increases and then decreases. The alloys reversibly absorb and desorb hydrogen at 298 K smoothly. When Co content is 0.2 at%, the hydrogen absorption capacity reaches the maximum value of 1.14 H/M, and the absorption capacities reach 1.09 H/M and 1.03 H/M in the first minute at 298 K and 323 K, respectively. Electrochemical performance measurement results show that La0.75Mg0.25Ni3.5?xCox alloys are completely activated within 2 cycles, and the cyclic stability of La0.75Mg0.25Ni3.3Co0.2 alloy approaches 63.7%after 100 charge/discharge cycles, which is higher than that (S100 = 60%) of La0.75Mg0.25Ni3.0Co0.5 alloy. Thus, the La0.75Mg0.25Ni3.3Co0.2 alloy exhibits optimum comprehensive properties of hydrogen storage and electrochemistry.