The dependence of the microporous structure of activated carbon fibers on the carbonization temperature of hydrate-cellulose fibers was studied by BET, TVFMP, and Horvath and Kawazoe techniques and methods of the nonlocal density functional theory. The surface energy distribution function was obtained, together with the micropore size distribution curves. It was shown that the effective micropore sizes calculated by the TVFMP method are close to the maximum of the distribution function found in terms of the theory of the density functional.