Ordered mesoporous carbon (CMK-3) was fabricated by a simple nanocasting method using SBA-15 as a hard template. The CMK-3 had a two-dimensional hexagonal mesoporous structure and a specific surface area of approximately 975.9 m(2) g(-1). The CMK-3 was modified by HNO3 solutions with magnetic stirring and ultrasonic activation, respectively, to explore the influence of various activation methods on pore structure, surface state, morphology, and electrochemical performance. The CMK-3 modified by ultrasonic activation (CMK-3-US) reached the optimal specific capacitance of 233.4 A g(-1) at 0.5 A g(-1) and retained 94.2% after 500 cycles in 3 M KOH electrolyte. Furthermore, a symmetric supercapacitor was successfully assembled using CMK-3-US electrodes, which delivered an excellent energy density of 21.5 Wh kg(-1) at a power density of 225 W kg(-1) and exhibited great long-term stability with 97.5% retention after 4000 cycles. Compared to magnetic stirring activation, ultrasonic cavitation could better increase the efficiency of the HNO3 activation for mesoporous carbon particles. The results indicate ultrasonic activation is an efficient way to modify carbon-based electrode materials for supercapacitors.