Abstract: As a byproduct of oil refining, petroleum coke with a high carbon content (about 90 wt%) has been shown to be a good raw material for porous carbons (PCs). PCs with high specific surface areas were derived from petroleum coke by KOH activation. The effect of KOH/coke mass ratio on the pore structure of the PCs and their electrochemical performance as electrodes of electric double layer capacitors were investigated. Results showed that the specific surface area and pore size distribution of the PCs could be efficiently controlled by the KOH/coke ratio. The pore sizes of the PCs increase with increasing KOH/coke ratio, and the largest specific surface area was as high as 2 964 m²·g^-1. A PC-5 electrode prepared with a KOH/coke ratio of 5:1 has a high specific surface area of 2 842 m²·g^-1 and mesoporosity of 67.0%, and has the largest specific capacitance at all investigated current densities among the PCs examined. This is ascribed to its high specific surface area and high mesoporosity. Hydrothermal modification of PC-3 (KOH/coke ratio at 3:1) in ammonia at 200℃ increases its specific capacitance, especially at high discharge current densities. This improved electrochemical performance can be attributed to nitrogen-doping that occurs during the process, and this can induce pseudo-capacitance and improve the hydrophilicity of the PC electrode to the electrolyte. KOH activation combined with ammonia hydrothermal modification is a simple yet efficient approach to prepare cost-effective PCs for supercapacitors with excellent electrochemical performance.