Granular micro/mesoporous carbon with a ratio of mesopore to total pore volume (V-meso/V-total) greater than 75% was prepared using coconut shells as a precursor by a one-step thermal treatment, i.e., combined pyrolysis and steam activation process. The process variables, such as final activation temperature, time, and water flow rate were studied. The N-2 adsorption isotherms of the samples were of type IV, indicating mesoporous characteristics. The mesoporosity of the resultant porous carbons prepared by this method is greater than the one of those prepared by the conventional two separate pyrolysis and activation processes. Experimental results showed that the yield of porous carbon was proportional to the final pyrolysis temperature and activation time. Additionally, with the increase of activation time and water flow rate, the mesoporosity increased considerably. When the activation time and water flow rate were kept constant, the mesoporosity also increased with a rise in the final pyrolysis temperature. Electrochemical tests indicate that with the increase of V-meso/V-total of the porous carbons, the equivalent series resistance (ESR) decreases and the capacitance retention is of 93% at a high current density of 5 A g(-1). Thereinto, the carbon electrode made from sample CS-800-0.12-60 with the highest V-meso/V-total have a high capacitance of 228 F g(-1) in 6.0 mol L-1 KOH electrolyte at 5 mV s(-1) and the energy density of 38.5 Wh kg(-1) with an ESR of 1.9 Omega at 0.5 A g(-1).