Abstract: The microstructural changes were analysed during the process of manufacturing carbon fibers by means of transmission electron microscopy (TEM),element analysis and X-ray diffraction(XRD).The skin-core structure which formed during the process of manufacturing polyacrylonitrile(PAN) precursors fibers was improved obviously,but did not disappear.The skin structure of fibers was compact while the core was loose.There was a kind of layer structure along the axial direction during the process of producing carbon fibers.In pre-oxidization and carbonization,there were strong chemical reactions and composition changes occurred inside fibers.