Abstract: Carbon nanotubes (CNTs) prepared by catalytical pyrolysis were activated by KOH to increase the specific surface area (SSA) and to modify the pore structures. The effects of the activation temperature and KOH/CNT ratio on the yields, SSA, crystal structure, microstructures and porous structures of CNTs were investigated by an adsorption method, XRD, TEM and HRTEM. Results showed that activation could effectively increase SSA and modify the pore structures of the CNTs. With an increase in activation temperature, SSA and the pore volume increased, while the yield decreased. Through activation, the inner hollow tubes of CNTs were released, and a large quantity of micropores and mesopores was created. With the increase of the KOH/CNT ratio, SSA and micropore volume increased to a maximum at a KOH/CNT ratio of 7∶1, while the yield decreased monotonically. The optimum activation condition to produce activated CNTs with high SSA was found at an activation temperature of 900℃ and a KOH/CNT ratio of 7∶1. SSA of CNTs activated at the optimal condition is 360.1m2/g, which is 14 times larger than that of the non-activated CNTs (24.5m2/g).