In this work, we substituted Mn2+ at the 4c site of LiFePO4 to prepare LiMn0.25Fe0.75PO4 in order to raise the working voltage. In order to study the phase transformation of lithium bi-metal phosphate (LiM′M′′PO4) during the lithiation/delithiation, in-situ synchrotron X-ray diffraction has been used. At 0.05C charge/discharge, X-ray patterns reveal that LiMn0.25Fe0.75PO4 undergoes two two-phase transformations during the delithiation, resulting from Fe2+/Fe3+ and then Mn2+/Mn3+ redox reactions. However, the phase transformation for lithiation is different, becoming a two-phase (Mn2+/Mn3+) reaction and single-phase (Fe2+/Fe3+) reaction. Even at a higher charge/discharge rate (0.5C), the results are the same. LiMn0.25Fe0.75PO4 also has a good cyclability, since there is no significant capacity fading during the cycling test. The X-ray patterns reveal that LiMn0.25Fe0.75PO4 still maintains a good crystal structure after 40 cycles because of its stable olivine structure.