Polyacrylonitrile (PAN)-based activated carbon fibers (A CFs) are amorphous graphiticcarbon, consisting of sp2 hexagonal carbon layers with different sizes of pores rangingfrom micropores to macropores. The microstructure, including graphite-like micro-crystal, surface chemistry, pore size distributions and so on, of PAN activated carbonfibers prepared by different activation methods were investigated by element onalysis(EA ), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and infraredspectra. The XRD patterns show that PAN-ACFs activated by different methods areof amorphous diffraction characteristic but composed of very small graphitic crystal-lites. The surface functional groups were analyzed by XPS and FT-IR spectra. Thesamples activated by air possess relatively abundant oxygen functional groups, and thesamples activated by potassium hydroxide possess less oxygen functional groups. Thisis attributed to the different activation mechanisms.