The structure evolution and thermal behaviors of polyacrylonitrile homopolymer (PAN) and terpolymer [P(AN-MMA-IA)] containing about 4.19 wt% methyl methacrylate (MMA), 0.98 wt% itaconic acid (IA) and 94.83 wt% acrylonitrile (AN) during heat treatments were studied by Fourier transform infrared (FTIR), differential scanning calorimetry and thermogravimetry. It was found that the presence of IA comonomer in PAN promoted the cyclization reactions significantly though initiating the cyclization reactions at lower temperature and proceeding at a faster rate than these of PAN homopolymer. Moreover, it was found that the dehydrogenation was based on the formation of cyclized structures. From the results of FTIR analysis, we found the MMA comonomer blocked the cyclization reactions to some extent. Extent of cyclization reactions at different temperatures was calculated through the data of FTIR. The sequence of reactions happened during the stabilization was oxidation > cyclization > dehydrogenation. The higher rate constant k implied that the ionic mechanism actually showed a kinetic advantage at low temperature over the free radical mechanism, even though the MMA comonomer inhibited cyclization reactions to some extent.