Activated carbon fibers (ACFs) are microporous systems comprised of a random network of nano-sized graphites. We investigated the structure and electronic properties of fluorinated ACFs in order to clarify the effects of fluorine-introducedsp3-carbon sites on the electronic state of the nano-sized π-electron system. The orbital diamagnetism is monotonically reduced with increasingF/C, suggesting that introducedsp3carbon sites make the extended π-conjugated system reduced. In the intermediate fluorine concentration region, the localized spin concentration is enhanced by fluorination, reaching a maximum atF/C∼ 0.8, whereas it tends to be lowered in the high concentration region. The enhancement in the spin concentration is explained in terms of the local destruction of the π-conjugated system at interior carbon sites by fluorine-inheritedsp3-carbon sites.