La0.75Mg0..25Ni3.5-xMnx (x=0,0.05,0.1,0.15,0.2) alloy ingots were prepared by induction melting. The effect of Mn substitution for Ni on the phase structure and the electrochemical characteristics of hydrogen storage alloys was studied. XRD analysis shows that the La0.75Mg0.25Ni3.5-xMnx consists of La2Ni7 phase, and LaNi5 phase with CaCu5 type appears when Mn is added. LaNi5 phase disappears with Mn higher content from x=0.15 to x=0.2. The electrochemical analysis shows that the substitutions lower the maximum electrochemical discharge capacity, the high rate dischargeability, and the exchange current density and have a little effect on the activation property. The replacements of Ni by Mn have a little effect on the cyclic stability and limiting current density (IL).