Heat-treatment of oxygen-containing petroleum coke (PC) at a high temperature (similar to 2800 degrees C) gave rise to closure of edge planes by carbon-carbon bond formation. To change the surface structure, surface modification of PC and those heat-treated at 1860, 2300, and 2800 degrees C (PC1860, PC2300, and PC2800) has been performed by ClF3 and NF3 at 200-500 degrees C. No surface fluorine was detected except in one sample treated by NF3, while small amounts of surface chlorine were found in all samples treated by ClF3. Small amounts of nitrogen were detected in two samples treated by NF3. Brunauer-Emmett-Teller surface areas and total mesopore volumes were reduced by surface modification. Transmission electron microscopic observation revealed that closed-edge planes of graphitized PC were destroyed and opened by surface modification with ClF3 and NF3. Removal of closed-edge planes increased first-charge capacities of many PC samples heat-treated at 1860-2800 degrees C by similar to 63 mAh/g (similar to 29.6%) at 150 mA/g.