Isotropic pitch based carbon fibers have been activated at different levels of burn-off by stema diluted in nitrogen. There is a slight uptake of CO₂ at 273K indicating that some microporosity is already present in the unactivated fiber. these micropores are, however, unaccessible to N₂ at 77K. The specific surface area and the micropore volume of carbon fiber were increased significantly upon activation, however after 60% burn-off, the micropore volume measured by D-R equation levels off where as the one determined by the αs-method steadily increase with the extent of activation. The adsorption of iodine and the decoloriza-tion of methylene blue of activated carbon fiber from aqueous solution were excellent, especially adsorption rate was greatly fast. The adsorption capacity was enhanced with the burn-off increasing, but it was also dependent on micropore size distribution.