The effect of magnesium content on the phase structure and electrochemical properties of Ml(1-x) Mg (x) Ni2.78Co0.50Mn0.11Al0.11 (x = 0.05, 0.10, 0.20, 0.30) hydrogen storage alloys was investigated. The results of X-ray diffraction reveal that all the alloys consist of the major phase (La, Mg)Ni-3 and the secondary phase LaNi5. With increase in x, the relative content of the (La, Mg)Ni-3 phase increases gradually, and the maximum capacity and low temperature dischargeability of the alloy electrodes first increase and then decrease. When x is 0.20, the discharge capacity of the alloy electrode reaches 363 mAh g(-1) stop at 293 K and 216 mAh g(-1) stop at 233 K, respectively. The high rate dischargeability of the alloy electrodes increases with increase in x. When the discharge current density is 1200 mA g(-1), the high rate dischargeability of the alloy electrodes increases from 22.0% to 50.4% with x increasing from 0.05 to 0.30. The cycling stability of the electrodes decreases gradually with increase in magnesium content.