La(1-x)MgxNi4.25Al0.75 (x=0.0, 0.1, 0.2, 0.3) hydrogen storage alloys were synthesized by means of a three-step induction melting. Their crystal structures and hydrogen storage properties were investigated in the present work. Studies inform that alloys with x=0.0 and 0.1 Mg contain a single LaNi4Al phase, however, those alloys with x=0.2 and 0.3 are composed of LaNi4Al, (La,Mg)Ni3 and AlNi3 phases. With x increasing from 0.2 to 0.3, the abundances of the secondary phases and the plateau pressures in the pressure-composition isotherms are lifted significantly. Meanwhile, their hydrogen storage capacities are remarkably reduced. The alloy with x=0.1 possesses the fastest absorption kinetics compared to the other alloys as well as a good hydrogen capacity and a low plateau pressure.