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The thermal behaviors of polyacrylonitrile precursor during thermal stabilization in inert gas were investigated by differential scanning calorimetry, thermomechanical analysis, and thermogravimetry. Combining these methods with the tracing of chemical changes by Fourier transform infrared spectroscopy indicated that complex reactions, including cyclization and pyrolytic reactions occurred sequentially. An imine-enamine tautomeric structure was formed at around 240 C and was converted to a conjugated structure when the temperature was increased to 400 C. A thermal stabilization mechanism was proposed and confirmed experimentally by using a two-step heating process. The apparent activation energies and the pre-exponential factors for these stabilization reactions were also estimated by the Kissinger, Ozawa, and ''Improved Coats-Redfern'' methods. To obtain a fit to the experiment data, a new kinetic model, named the ''Three Regions Kinetic Model,'' was proposed using the Improved Coats-Redfern method. The applicability of this model and the prediction of the stabilization profile at a given heating rate were verified by plotting conversion rate against conversion profiles.