The electrochemical properties and phase stability of the multi-component olivine compound LiMn1/3Fe1/3Co1/3PO4 are studied experimentally and with first-principles calculation. The formation of a solid solution between LiMnPO4, LiFePO4, and LiCoPO4 at this composition is confirmed by XRD patterns and the calculated energy. The experimental and first-principle results indicate that there are three distinct regions in the electrochemical profile at quasi-open-circuit circuit potentials of similar to 3.5V, similar to 4.1V, and similar to 4.7V, which are attributed 50 Fe3+/Fe-2+,Fe- , Mn3+/Mn2+, and Co3+/Co2+ redox couples, respectively. However, exceptionally large polarization is observed only for the region near 4.1V of Mn3+/Mn2+ redox couples, implying; an intrinsic charge transfer problem. An ex situ XRD study reveals that the reversible one-phase reaction of Li extraction/insertion mechanism prevails, unexpectedly, for all lithium compositions of LixMn1/3Fe1/3Co1/3PO4 (0 <= x <= 1) at room temperature. This is the first demonstration that the well-ordered non-nanocrystalline (less than 1% Li-M disorder and a few hundred nanometer size particle) olivine electrode can be operated solely in a one-phase mode.