A commercial activated carbon fiber with micropores, CH700-20 (Kuraray), was reformed to a mesoporous one by catalytic gasification. The catalytic gasification was composed of two steps : CO₂ pretreatment and air oxidation. Cobalt was used as the catalyst and gasification was performed in the temperature rangen of 500-700℃. BET surface area and pore volume of catalytically modified carbon fiber were analyzed by N₂ adsorption. BET surface area of the original CH702-20 was 1,711㎡/g, and the mesopore volume percentage was 11.9%. After catalytic gasification, BET surface area was similar to the original CH700-20, while mesopore volume percentage increased up to 56%. The average pore size of mesopores was 3-4nm in diameter. The average size of mesopores could be controlled with nanometer resolution by varying the temperature and time of activation.