Olivine compounds LiFe1−x Mn x PO4 (0.0< x <0.4) were prepared by the solid-state reaction, and the electrochemical properties were studied in order to examine the effects of Mn2+-substitution. The substitution led to the modification of the electrochemical performance, such as initial capacity, capacity fading and polarization. From the cyclic voltammetry, it was found that the effective Li+ ionic diffusion coefficient was always larger in the charging process than in the discharging process and that it became larger with an appropriate amount substitution. The structural analysis on the chemical-delithiated compounds exhibited anomalous expansion of the unit cell along c-axis with the substitution, while the lattice parameters of the pristine compounds obeyed Vegard's law. The relationship between the Li+ ion diffusion and the bottle-neck area of (010) zigzag path was discussed. From the results, it was considered that Mn2+ had no direct contribution on the electrochemical reaction but influenced both electronic and ionic conductivities, which led to some modifications in the electrochemical performance.