Three polyacrylonitrile fibers were oxidative stabilized by two heat treatment modes. OM and SEM were utilized to analyze the microstructure of oxidative stabilized fibers. In experiment I (by stepwise increasing temperature), no skin-core structure occurs in the polished cross-sections of stabilized fibers from 195 degrees C to 265 degrees C, unless they are subsequently heated by temperature higher than 270 degrees C. In experiment II (by isothermal temperature), the oxidative stabilized fibers heated at 230 degrees C for 120min shows homogeneous microstructure, but those heated at 250 degrees C for only 60min displays distinct skin-core morphology. The shape of the boundary between the skin and the core resembles the cross-section shape of precursor fibers, but the thickness of the skin keeps almost unchanged of about 4 mu m. With the increasing of temperature, the stiffness of stabilized fibers increases and the evolution of microstructure undergo four steps. It indicated that temperature is the essential factor that affects the skin-core morphology, while the cross-section shape of precursor is irrelevant to that. If fibers are directly heated at high temperature without low temperature treatment, the light colored core will fuse into a hole and deteriorate the properties of carbon fibers.