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-introduced sp(3)-carbon sites on the electronic state of the nano-sized pi-electron system. The orbital diamagnetism is monotonically reduced with increasing F/C, suggesting that introduced sp(3) carbon sites make the extended n-conjugated system reduced. In the intermediate fluorine concentration region, the localized spin concentration is enhanced by fluorination, reaching a maximum at F/C similar to 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 pi-conjugated system at interior carbon sites by fluorine-inherited sp(3)-carbon sites.