The influence of interface structure between Nafion(R) and an electrode surface on cathodic oxygen reduction has not been fully clarified yet in spite of the need for improving the performance of polymer electrolyte fuel cells (PEFCs) by clarifying the influence. In the present study, oxygen reduction was investigated at the interface between Nafion(R) and a glassy carbon (GC) surface where the ratio of the fluorinated region in Nafion(R) contiguous with the GC surface was controlled by electrochemical oxidation of the GC surface. Measurements of the oxygen reduction current were carried out using rotating GC disk electrodes and those covered with Nafion(R), and the kinetic currents were determined for these electrodes. The surface oxidation resulted in an increase in the oxygen reduction current both at bare GC and Nafion(R)-coated GC; thus, the ratio of the kinetic current at the latter to that at the former was used for evaluating the effect of change in the interface structure between Nafion(R) and the GC surface. The net current ratio that was calculated by dividing the kinetic current ratio by the ratio of the GC surface contiguous with the ionic cluster region, where electrode reactions occur at the Nafion(R)-coated electrodes, increased with a decrease in the blocking of the GC surface by the fluorocarbon region; while excessive reduction of the region reduced the ratio. This result indicates the possibility of optimization of the Nafion I electrode interface for O-2 reduction by controlling the ratio of the electrode surface contiguous with the fluorocarbon region.