The porous structure of pitch-based carbon aerogels (P-CAs) can be modified by KOH activation. It is found that decreasing the carbonization temperature of precursor CAs and increasing the mass ratio of KOH to CAs help to the formation of 0.7nm-sized micropores and 2.7nm-sized mesopores, respectively. The origin and the pore size of micropores play an important role in controlling electrochemical properties. The carbonization-forming micropores have stronger energy storage efficiency than activation-forming micropores, and only those with diameter below 1nm (Microp<1nm) are the crucial place to storage energy. Due to the substantive increase of the number of the Microp<1nm, the highest specific capacitance of the as-prepared activated samples can reach 187.2Fg−1 at 5mAcm−2, 1.8 times as large as that of their precursor CAs. Furthermore, this capacitance is still up to 173.3Fg−1 when increasing the current density to 50mAcm−2, indicating that the activated samples have a high-rate charge–discharge performance.