Isotropic pitch-based carbon fiber was isothermally activated in CO₂ atmosphere. Structural parameters of the isotropic carbon fibers and activated carbon fibers (ACFs) were evaluated by X-ray different (XRD). The d₂and La of the carbon fibers were measured to 4.04 A and 23.6A and those ACFs were 4.29A and 22.7A, respectively, representing less orderd through activation process.