Abstract Activated carbon fibers (ACF) consist of microporous carbon with a huge specific surface area (SSA) ranging from 700 m2 g−1 to 3000 m2 g−1, and a having random structures consisting of an assembly of micrographites with a dimension of ca. 20 × 20 Å2. The electrical conductivity and magnetic susceptibility were investigated for ACFs with SSA = 1000 and 2000 m2 g−1 in order to clarify the relation between the electronic properties and the structure of ACF having a random network of micrographites. The electrical conductivity is explained by the two-dimensional variable-range hopping conduction at lower temperatures and thermally activated conduction at higher temperatures. The introduction of N2 or O2 gas to a sample induces a change in the conductivity, which is considered to be caused by a structural change and a charge transfer between dangling bonds and O2 gas. The observed value of the orbital diamagnetic susceptibility is considerably small compared with that of a condensed polycyclic aromatic hydrocarbon having the same dimensions as that of the micrographite in ACF. This implies that the micrographitic domains have a deformed planar structure with the presence of defects.