In order to develop cathode materials for Ni-MH batteries with high energy density,La_0.79Mg_0.21Ni_3.95 hydrogen storage alloy was prepared by vacuum induction melting,and the phase composition,microstructure and electrochemical properties of as cast and annealed hydrogen storage alloys were compared and analyzed.The results showed that there were only LaNi₅ and（La,Mg）₂Ni₇ phases in as cast and 800 ℃/24 h annealed La_0.79Mg_0.21Ni_3.95 hydrogen storage alloys.When the tem-perature was raised to 900 ℃ or above,different contents of（La,Mg）₅Ni₁₉ phase and（La,Mg）₆Ni₂₄ phase were formed in the hydrogen storage alloy.The reversible hydrogen absorption and desorption performance of 900 ℃/24 h annealed hydrogen storage alloy was higher than that of 950 ℃/48 h annealed hydrogen storage alloy.The maximum discharge capacity of as cast and annealed hydrogen storage alloys reached the maximum in the first three cycles,and the hydrogen storage alloy annealed at 950 ℃/48 h mainly consisted of（La,Mg）₆Ni₂₄ phase,which had high cycle stability.As cast and annealed La_0.79Mg_0.21Ni_3.95 hy-drogen storage alloys had good electrochemical activation properties.The order of HRD₁₅₀₀ from high to low was: 950 ℃/48 h>950 ℃/24 h>900 ℃/24 h>800 ℃/24 h>as cast,the results showed that the HRD₁₅₀₀ of hydrogen storage alloy had the same trend with that of hydrogen diffusion rate（D） and exchange current density（I₀）.The maximum HRD₁₅₀₀ of hydrogen storage alloy annealed at 950 ℃/48 h was mainly due to the fact that the alloy electrode contained 61.8% of（La,Mg）₆Ni₂₄ phase and had higher D and I₀.