Activated carbon fibers(ACF) show different activation behaviors and adsorptive properties, which depend on the activating conditions and starting materials used. So, PAN- and pitch-based activated carbon fibers were prepared by flow rates of varying the activating agents(CO₂, and steam) and temperatures in activation process. The pitch-based activated carbon fibers were microporous materials of high specific surface areas(>2,500 m²/g). The reactivity in steam as activating agent was about three times faster than in CO₂, the reactivity of pitch-based carbon fibers as starting materials was higher than PAN-based carbon fibers. The ranges of average pore diameter for steam and CO₂ were about 15-17Å and 11-13Å, respectively. It implies that smaller and more uniform micropores are developed in CO₂. Optimum conditions of the activation process for preparing high quality ACF are pitch based carbon fiber as starting material, lower temperature(800℃), CO₂ as activating agent, flow rate of 38 ml/hr.