The electrochemical reaction mechanisms of tridymite FePO4 with Li ions were studied in the voltage range of 2.4–0V. The lithiation/delithiation of FePO4 was found to be similar to that of binary 3d-metal oxides, which involve the formation and decomposition of Li2O along with the reduction and oxidation of nanoscale 3d-metal nanoparticles. In the first discharge, FePO4 was reduced to metallic Fe nanoparticles dispersed in a lithium compound matrix consisting of Li3PO4 and Li2O. In the subsequent charge, these metallic Fe nanoparticles were partially oxidized to FeO by the decomposition of Li2O. The Li3PO4 matrix was inactive during the change of potential from 0 to 2.4V. It is notable that the re-oxidation of metallic Fe occurred only in the case of small nanoparticles, and the large Fe particles remained electrochemically inert during the reversible reaction.