Charge/discharge processes of organic radical batteries based on the radical polymer's redox reaction are largely influenced by carbon fibers consisting in the composite electrodes to help electron transfer. To find the optimal structure of the composite electrodes, the dominant electron transfer processes were determined by ac impedance measurement of the composite electrodes. A strong correlation between the overall electron transfer resistance of the composite electrodes and the materials of the current collector suggests that the electric conduction to the current collector through the contact resistance should be crucial. It was also confirmed that the charge/discharge performance of the composite electrode was related to the overall electron transfer resistance of the composite electrode. These results indicated that the charge/discharge performance of the radical battery was dominated by the interfacial electron transfer processes at the current collector/carbon fiber interface and that the rate performance would be much improved by suitably designing the interfacial structure.