The effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of the annealed La_(1-x) Mg_x Ni_3.5(x=0.20, 0.23, 0.25 at%) alloys have been studied. All the samples were mainly composed of(LaMg)_2Ni_7,(LaMg)Ni_3, and LaNi_5 phases. Mg substitution for La changed the phase abundance, but did not change the constitution of all phases, which is con fi rmed by the results of backscattered SEM images and EDS analysis. The P–C isotherms indicated that the La_(1-x) Mg_x Ni_3.5alloys reversibly absorbed and desorbed hydrogen smoothly at 298 K. The hydrogen absorption cyclic stabilities of La_(1-x) Mg_x Ni_3.5alloy after 5 hydrogen absorption/desorption cycles reached the maximum values of91.9% and 96.0% at 298 K and 323 K, respectively. The hydrogen desorption capacity and plateau pressure for the La_(1-x) Mg_x Ni_3.5alloy reached the maximum values of 1.055 H/M and 0.074 MPa, respectively. The desorption capacities of La_(1-x) Mg_x Ni_3.5reached 0.193 H/M and 0.565 H/M in the fi rst minute at 298 K and 323 K, respectively. Electrochemical property measurement indicated that La_(1-x) Mg_x Ni_3.5(x=0.20,0.23, 0.25 at%) alloys possessed excellent activation capability and were completely activated within3 cycles. Discharge capacities of La_(1-x) Mg_x Ni_3.5alloys reached 378.2 m A h/g(x = 0.20 at%), 342.7 m A h/g(x = 0.23 at%), and 369.6 m A h/g(x = 0.25 at%), respectively. Moreover, energy density of La_(1-x) Mg_x Ni_3.5alloy was much larger than that of La_0.80Mg_0.20Ni_3.5 alloy and nearly approaches the maximum value of La0.75Mg0.25Ni3.5. Thus, the La_(1-x) Mg_x Ni_3.5alloy exhibits optimum comprehensive properties of hydrogen storage and electrochemistry. The effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of the annealed La 1- (1-x) Mg_x Ni_3.5 (x = 0.20, 0.23, 0.25 at%) alloys have been studied. of (LaMg) _2Ni_7, (LaMg) Ni_3, and LaNi_5 phases. Mg substitution for La changed the phase abundance, but did not change the constitution of all phases, which is con fi rmed by the results of backscattered SEM images and EDS analysis. The P-C isotherms indicated that the La_ (1-x) Mg_x Ni_3.5 alloys reversedbly absorbed and desorbed hydrogen smoothly at 298 K. The hydrogen absorption cyclic stabilities of La_ (1-x) Mg_x Ni_3.5alloy after 5 hydrogen absorption / desorption The hydrogen desorption capacity and plateau pressure for the La_ (1-x) Mg_x Ni_3.5 alloy reached the maximum values ​​of 1.055 H / M and 96.0% at 298 K and 323 K respectively 0.074 MPa, respectively. The desorption capacities of La_ (1-x) Mg_x Ni_3.5reached 0.1 93 H / M and 0.565 H / M in the fi rst minute at 298 K and 323 K, respectively. Electrochemical property measurement indicated that La - (1-x) Mg_x Ni_3.5 (x = 0.20, 0.23, 0.25 at%) alloys Discharge capacities of La_ (1-x) Mg_x Ni_3.5alloys reached 378.2 m A h / g (x = 0.20 at%), 342.7 m A h / g (x = 0.23 at %), and 369.6 mAh / g (x = 0.25 at%), respectively. Moreover, energy density of La_ (1-x) Mg_x Ni_3.5alloy was much larger than that of La_0.80Mg_0.20Ni_3.5 alloy and nearly, the maximum value of La0.75Mg0.25Ni3.5. Thus, the La_ (1-x) Mg_x Ni_3.5alloy exhibits optimum properties of hydrogen storage and electrochemistry.