The pore structures of commercial pitch and PAN-based activated carbon fibers (ACFs) were investigated. The pore size and pore size distribution Of pitch-based ACFs were measured by nitrogen adsorption isotherms and Xe-129 NMR spectroscopy and compared with each other. Scanning tunneling microscopy showed that the ACFs were composed of spherical microdomain units the size of a few nanometers. The activation mechanism of ACFs was considered and explained by novel hypothesis; the concept of microdomain structure of ACFs was considered and explained to overcome limitation of the conventional fractal hypothesis. Whereas micropores were generated on each microdomain, the origin of mesopores was interdomain pores, resulting from the microdomain hypothesis.