Olivine LiFePO4 particles were prepared by solid-state reaction using Li2CO3, (NH4)2HPO4 and FeC2O4 2H2O as raw materials, and they were coated with an appropriate amount of carbon through thermal decomposition of C16H10 pyrene. Cathodes using the olivine particles were subjected to an open-circuit voltage measurement under the relaxation condition of 24h at each SOC and DOD states. The electrochemical reaction in the LiFePO4 cathode was composed of a large plateau around 3.45V with sloped regions nearby for both the fully charged and discharged states. It was found that the sloped region widths exhibited a hysteresis, that is, they depend on the direction of the redox reaction. Furthermore, both sloped regions became narrower when the operating temperature was raised from 30°C to 60°C. These facts implied that the obtained profiles were not in an equilibrium state with a quasi-OCV profile than the real one, and that the potential relaxation in the sloped regions took an extremely long time (more than 24h).