Activated carbon fibers (ACFs) were surface modified with oxygen plasma at low pressure. The novel effects of the plasma treatment on the microstructural properties of the ACFs were characterized using the Brunauer, Emmett, and Teller method and scanning electron microscopy. Micropores developed on the ACFs. Moreover, the specific surface area and micropore volume increased by 10% at a certain plasma treatment time and power. The changes in the structural properties of the ACFs are discussed in detail with the respect of plasma etching. X-ray photoelectron spectroscopy revealed new oxygen-containing groups, such as CsO, CdO, and OsCdO, had formed on the surface of the ACFs after plasma treatment. Plasma surface oxidative reactions such as the generation of radicals, the combination of the radicals and active oxygen species in the plasma chamber, and the generation of the various oxygen-containing groups are believed to have occurred. The effect of the plasma treatment parameters such as plasma treatment time and power was examined from the perspective of both surface structure and chemistry. It was observed that the micropores and surface functionalities of the ACFs were increased under moderate treatment conditions (50 s and 100 W).