Polybenzoxazine, a high-perfomance phenolic resin, was used as a precursor for carbon aerogel electrode of supercapacitors. A cost-effective ambient drying was used for benzoxazine aerogel preparation. After the pyrolysis of benzoxazine aerogel under a nitrogen atmosphere, carbon aerogel was obtained. The BET surface area of the carbon aerogel was approximately 360 m2/g. The activation of the carbon aerogels was also investigated in order to compare the physical and electrochemical properties. The BET surface area of the activated carbon aerogels increased more than twice in comparison with the unactivated one. The electrochemical behaviors were studied by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The results showed that the polybenzoxazine-derived-carbon aerogel exhibited good electrochemical performance. A specific capacitance of the carbon aerogel electrode was 30 F/g obtained at current density 5 mA/cm2. The specific capacitance increased to 78 F/g at the same current density after carbon dioxide activation. Moreover, the best electrochemical behaviors with the specific capacitance of 109 F/g were obtained from the electrode prepared from carbon aerogel which underwent heat treatment at 300 °C in air. The electrochemical impedance spectroscopy as well as cyclic voltammetry was also confirmed those electrochemical behaviors.