Homogeneous-sized LiMPO(4) (M = Fe, Mn) nanorods and bulk particles were synthesized, and the thermodynamic characteristics of their mixtures as electrodes were analyzed to study the lithiation/delithiation mechanism for the general case of nanoparticles with a heterogeneous particle size distribution. We show that ionic transport occurs between nano and bulk particles in a cell at equilibrium, due to their electrochemical potential difference that originates from their different thermodynamic properties. This means that one phase in a single particle is preferred to two phases in a single particle during lithiation/delithiation of LiMPO(4) from the viewpoint of thermodynamics if the electrode is composed of differing particle sizes.