In this study, a combination of physical and chemical activation was used to produce activated carbon from sunflower stalks. The NaOH activated carbon possess a high specific surface area of 2658 m2/g. The micropore fraction and surface area obtained is much higher than a commercial activated carbon. The electrodes from the activated carbons were electrochemically analyzed in a two-electrode supercapacitor cell with 1 M H2SO4 electrolyte. The results show that the high surface area of sunflower activated carbon resulted in significantly high specific capacitance of 207 F/g at 0.05 A/g current density. Moreover, a high energy density of 18.4 Wh/kg was obtained at the power density of 80 W/kg. The results also showed the importance of pore structure on the supercapacitor performance.