High-surface-area polyacrylonitrile (PAN) activated carbon fibers having different pore size distribution activated by KOH were investigated. Nitrogen adsorption, XRD, SEM, and TEM were used to characterize the microstructure of PAN-ACFs. The specific surface area of samples was calculated from the standard BET method, and micropore surface area and volume were obtained from the Horvath-Kawazoe equations. The average pore size and characteristic energy were calculated by the Dubinin-Radushkevich equation according to the multistage adsorption mechanism. The whole pore size distribution was calculated by employing the regularization method according to the density functional theory, which is based on a molecular model for the adsorption of nitrogen in porous solids. It was shown that the isotherms were type I, the pore size was around 0.4-0.8 nm, and the mesorpore size was around 2-4 nm. The XRD pattern showed that PAN-ACFs activated by KOH are of amorphous material composed of very small crystallites. The SEM and TEM results showed that the monograph differs with differing activation degree, and the network is uniform or disordered. That all of these methods are in good agreement with one another.