La-Mg-Ni-based La0.75Mg0.25Ni3.3Co0.5 hydrogen storage alloy was synthesized by high-energy mechanical milling blending of the La0.75Ni3.3Co0.5 as-cast alloy prepared by vacuum arc melting and elemental Mg, and subsequent isothermal annealing. The chemical compositions, microstructures and electrochemical properties of the as-cast La0.75Ni3.3Co0.5 alloy, the milled and annealed La0.75Mg0.25Ni3.3Co0.5 alloys were investigated, respectively, by inductively coupled plasma, X-ray diffraction, differential scanning calorimetry, X-ray photoelectron spectroscopy and electrochemical measurements. The results show that single LaNi5 phase exists in the as-cast La0.75Ni3.3Co0.5 alloy. The milled La0.75Mg0.25Ni3.3Co0.5 alloy contains multiphase structure, besides the main LaNi5 phase, a small amount of (La,Mg)Ni-3 and (La,Mg)(2)Ni-7 new phases are observed as well. The annealed La0.75Mg0.25Ni3.3Co0.5 alloy is composed of LaNi5 and (La,Mg)(2)Ni-7 phases. Annealing treatment can result in (La,Mg)Ni-3 phase converting into (La,Mg)(2)Ni-7 phase. The electrochemical measurements indicate that the maximum discharge capacity and discharge potential characteristic of the as-cast La0.75Ni3.3Co0.5 alloy are better than those of the milled La0.75Mg0.25Ni3.3Co0.5 alloy, whereas worse than those of the annealed La0.75Mg0.25Ni3.3Co0.5 alloy. The cyclic stability of the milled La0.75Mg0.25Ni3.3Co0.5 alloy is slightly better than that of the as-cast La0.75Ni3.3Co0.5 alloy, whereas obviously worse than that of the annealed La0.75Mg0.25Ni3.3Co0.5 alloy. Overall, the annealed La0.75Mg0.25Ni3.3Co0.5 alloy performs the best in the maximum discharge capacity, discharge potential characteristic and cycling stability. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.