The microstructure of the polyacrylonitrile (PAN)-stabilized fibers was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). XRD analysis indicated that the chemical reactions sped up notably when temperature was above 250 degrees C. The crosssection SEM image of the stabilized fibers presented concentric multilayer morphology. TEM images showed that the lamellar-like skin was compact and the core was disorderly and loose. The vermiculate-like crystallites in the precursor fibers were transferred to spheres during stabilization process. HRTEM images showed that many spheres existed in the stabilized fibers. The exterior of spheres which had annular structure was amorphous phase. The inner of spheres which was not completely stabilized had some crystallite areas. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2413-2417, 2009