Petroleum coke (PC) was modified by HClO 4 oxidization. Activated carbons (ACs) were prepared using a mass ratio of KOH/modified PC of 3:1 (denoted OAC-3) and a KOH/PC mass ratio of 4:1 (denoted AC-4) for comparison. XRD, I 2 adsorption, N 2 adsorption and cyclic voltammograms were used to investigate the influence of HClO 4 oxidization on the structure of the PC and the performance of the resultant AC. Results indicated that HClO 4 oxidization increased the d (002) of microcrystalline PC from 0.344 to 0.353nm and decreased L c from 2.34 to 1.75nm. The specific surface areas of AC-4 and OAC-3 were 2 929 and 3 058m 2 /g, respectively. AC-4 and OAC-3 had a specific gravimetric capacitances of 361.3 and 392.7F/g at a scan rate of 0.5mV/s, respectively. A supercapacitor based on OAC-3 possessed a better power performance than that based on AC-4.[New Carbon Materials 2013, 28(4): 262–266]