Abstract: By the comparative differential scanning calorimetry (CDSC) method, the heat evolution of the acrylonitrile (AN)/itaconic acid (IA)/methl acrylate (MA) copolymeric fibre during the initial stages of pyrolysis was studied in nitrogen atmosphere. From the CDSC measurements, it was observed that the sharp intense exotherm, occurring on a temperature-programmed pyrolysis, was related to the reaction of nitrile groups in the fibre. A first order kinetic equation was deduced for the thermal degradation of the AN/IA/MA copolymeric fibre studied. Relevant kinetic parameters of exo-thermic process were obtainedand discussed. For the sample of Jilin Research Institute of Chemical Technology, it was shown that cyclization occurred mainly; the cyclization activation energy was calculated to be 91.030kJ/mol. and the apparent activation energy was determined. Wherefore, we believe that the results are valuable and may play an important role in the improvement of the operating condition of the stabilization of polyacrylonitrile (PAN) precursors on the industrial scale.