Abstract: The gradient warming and isothermal oxidative stabilization of polyacrylonitrile(PAN) fibers were carried out, and the formation mechanism of skin-core structures and pervasion of oxygen elements of PAN fibers at different stabilization temperatures were investigated systematically by means of FTIR, ¹³C-NMR, EA, DSC, energy dispersive analysis system of X-ray (EDS) and density. The study indicates that the lower the oxidation rate than the diffusion rate, the greater is the difference between outer and core of PAN fiber cross-section, and the morphologies of skin-core structure will appear on the cross-section simultaneously. The formation of skin-core structures of the preoxidized fibers is not only relevant to the temperature, but also highly relevant to the heating mode. By using EDS, the radial pervasion of oxygen elements of preoxidized fibers at different temperatures can be effectively quantified, the oxygen uptake on the different regions of the cross-section of stabilized fibers also can be visually displayed.