The effect of partial substitution of Mn for Ni on the structural and electrochemical properties of the La0.7Mg0.3Ni2.975-xCo0.525Mnx (x=50.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has been investigated systematically. The results of X-ray powder diffraction and Rietveld analyses showed that all alloys consisted of the (La, Mg) Ni3 phase and the LaNi5 phase, and the content of the (La, Mg) Ni3 phase first remained unchanged (～77wt%) and then decreased, but the content of the LaNi5 phase increased progressively with increasing x. Meanwhile, the lattice parameters and cell volumes of the (La, Mg) Ni3 phase and the LaNi5 phase all increased with increasing Mn content. The pressure composition isotherms showed that the hydrogen storage capacity first remained almost unchanged and then decreased with increasing x from 0.0 to 0.5, and the equilibrium pressure decreased from 0.51atm to 0.06atm. The electrochemical measurements indicated that the maximum discharge capacity first remains unchanged (～400mAh/g) with increasing x from 0.0 to 0.2 and then decreased when x increased further. Moreover, the high rate dischargeability, the exchange current density I0, the limiting current density IL, and the hydrogen diffusion coefficient D of the alloy electrodes all increased first and then decreased with increasing x, which indicates that the kinetics of hydriding/dehydriding of the La0.7Mg0.3Ni2.975-xCo0.525Mnx (x=50.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys increased first up to x=0.1 and then decreased with further increasing x.