The electron beam was imposed on the polyacrylonitrile precursor fibers before the fibers were stabilized. The effect of electron beam irradiation on the chemical structure, transverse section, and surface morphology and thermal properties of the fibers in the process of stabilization was characterized by the use of Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and differential scanning calorimeter, respectively. A parameter eta = I(C=N)/[I(C=N) + I(C N)] was defined to evaluate the extent of cyclization in the stabilization process. The kinetic parameters, viz. activation energy (E) and pre-exponential factor (A) of the stabilization reactions, were calculated by Kissinger method. FTIR analysis indicated that the cyclization of nitrile groups was initiated at room temperature by electron beam irradiation. The transformation of C N groups to C=N ones was accelerated in the process of stabilization. The extent of cyclization of the stabilized fibers was increased. SEM analysis indicated that irradiation could also decrease the internal and surface defects of the stabilized fibers treated at 300 degrees C. The activation energy of cyclization reaction was reduced from 302 to 280 kJ/mol and 260 kJ/mol through 100 and 200 kGy electron beam irradiation, respectively. The reaction temperature range was expanded, and the exothermic rate was slowed down in the process of stabilization, which was the reason why the stabilized fibers have improved cyclization degree and less internal defects. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 90-96, 2011