Self-discharge characteristics of synthetic graphite electrodes with different particle sizes have been investigated by electrochemical and analytical methods. Variations of open-circuit potential (OCP), residual capacity and electrochemical impedance responses were monitored during or after the storage at different temperatures. The rate of self-discharge depended on the particle size, or the specific surface area, of the graphite sample. The temperature dependence of the self-discharge rate revealed that the activation energy for the process does not depend on the particle size of the graphite electrode. The ac impedance responses suggest that the self-discharge processes consist of the loss of the Li species in the graphite and SEI (Solid-Electrolyte-Interphase) formation at the graphite surface. The latter SEI formation process was found to be dependent on the surface site, edge or basal planes, from the results of surface observation and elemental analyses.