The microporous structure of activated carbon fibers is studied in terms of the BET theory, the theory of volume filling of micropores, the Horvath-Kawazoe theory, and the nonlocal density functional theory. It is shown that effective micropore sizes, as calculated within the framework of the theory of volume filling of micropores, lie in the region of the maximum of the pore volume distribution function calculated by the density functional theory, whereas the distributions found via the Horvath-Kawazoe theory are shifted to the region of fine pores.