Microporous activated carbon fibers (ACF) having huge specific surface areas are characterized as a three-dimensional network of nano-sized graphite domains. We investigate ACF-hosting host-guest systems aiming at novel phenomena produced by the host-guest interface interactions. An extraordinarily large amount of helium is condensed in the ACF micropores, suggesting the presence of ultra-micropores and the enhancement in the helium- nano-graphite interaction. Chemisorbed oxygen molecules are stabilized in the singlet states caused by the interaction with nano-graphite and it makes the micorpore volume swell. Iodine forms an intercalation system with nano-graphites through charge transfer process, in contrast with the behavior of bulk graphite that does not accept iodine as an acceptor-type intercalate. Fluorine reacting with nano-graphite generates dangling bond spins by breaking graphite π- bonds, where the fluorine-induced defect is found to have a widely extended electronic structure.