Isotropic pitch-based carbon fiber was activated with the mixture of nitrogen and super heated water vapor to increased the specifc surface area and some activated carbon fiber shows a specific surface area of 2020cm²/g. The increase of specific surface area seems to be caused by the enlargement of micropores of fiber surface to meso-pores, and it is believed that parts of iinner pores have been exposed to outside. The weight loss of carbon fiber was pro-portional to activation time and temperature. The specific surface area of carbon fiber was linearly proportional to the weight loss at constant activation temperature.