A low-cost rout for modification the polyacrylontrile(PAN) precursor fibers was developed.The approach involved pretreatment PAN precursor fibers with UV irradiation for various periods of time before the fibers were stabilized.The effect of UV irradiation on the chemical structure,orientation factor,density,crystallite size and morphology of the fibers in the process of stabilization was characterized by use of fourier transform infrared spectroscopy(FTIR),float-sink procedure,X-ray diffraction(XRD),scanning electron microscope(SEM),respectively.The results showed that UV irradiation could increase the density of the fibers in stabilization process.FTIR analysis indicated that the cyclization of nitrile groups was initiated at room temperature by UV irradiation.The transformation of C≡N groups to C=N ones was accelerated in the process of stabilization.The orientation factor of irradiated fibers was also increased.The crystallite size was decreased at first and increased later,and the better irradiation time of UV was 3 min according to the XRD test.SEM analysis indicated that irradiation could decrease the internal and surface defects of the stabilized fibers treated at 300 ℃. A low-cost rout for modification of polyacrylontrile (PAN) precursor fibers was developed. The approach involved pretreatment of PAN precursor fibers with UV irradiation for various periods of time before the fibers were stabilized. The effect of UV irradiation on the chemical structure, orientation factor , density, crystallite size and morphology of the fibers in the process of stabilization was characterized by use of fourier transform infrared spectroscopy (FTIR), float-sink procedure, X-ray diffraction (XRD), respectively. The results showed that UV irradiation could increase the density of the fibers in stabilization process. FTIR analysis indicated that the cyclization of nitrile groups was initiated at room temperature by UV irradiation. The transformation of C≡N groups to C = N ones was accelerated in the process of stabilization. The orientation factor of irradiated fibers was also increased. crystallite size was decreased at first and increased later, and t he better irradiation time of UV was 3 min according to the XRD test. SEM analysis that the could could decrease the internal and surface defects of the stabilized fibers treated at 300 ° C.