Abstract: Petroleum coke (PC) was modified by HClO₄ 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, I₂ adsorption, N₂ adsorption and cyclic voltammograms were used to investigate the influence of HClO₄ oxidization on the structure of the PC and the performance of the resultant AC. Results indicated that HClO₄ oxidization increased the d₍002) of microcrystalline PC from 0.344 to 0.353 nm and decreased L_c from 2.34 to 1.75 nm. The specific surface areas of AC-4 and OAC-3 were 2 929 and 3 058 m²/g, respectively. AC-4 and OAC-3 had a specific gravimetric capacitances of 361.3 and 392.7 F/g at a scan rate of 0.5 mV/s, respectively. A supercapacitor based on OAC-3 possessed a better power performance than that based on AC-4.