Structural features and electrochemical performances of cokes pyrolized from oxidized cokes were examined, and compared with KOH-activated coke. Needle cokes (d002 = 3.5 Å), having a graphene layer structure, were changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an NaClO3/needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized coke was expanded to 6.9 Å with increasing oxygen content. After heating at 200 ℃, the oxidized coke was pyrolized to the graphene layer structure with inter-layer distance of 3.6 Å. However, the change of the inter-layer distance of the needle coke was not observed in the KOH activation process. On the other hand, an intercalation of electrolyte ions into the pyrolized coke, observed at first charge, occurred at 1.0 V, in which the value was lower than that of KOH-activation coke. The cell capacitor using pyrolized coke exhibited a lower internal resistance of 0.57 Ω in 1 kHz, and a larger capacitance per weight and volume of 30.3 F/g and 26.9 F/ml at the two-electrode system in the potential range 0∼2.5 V than those of the cell capacitor using KOH-activation of coke. This better electrochemical performance may be associated with structure defects in the graphene layer derived from the process of the inter-layer expansion and shrinkage.