Abstract: The structure evolution of polyacrylonitrile(PAN) precursor fibers during stabilization process was studied systemically by density,fourier transform infrared spectroscopy(FT-IR),wide-angle X-ray diffraction(WXRD),differential scanning calorimeter(DSC) and scanning electron microscope(SEM).The results showed that the density and relative cyclization index of PAN fibers was increased,the crystallite size was increased at first and decreased later and the crystallinity was decreased,which manifested that the cyclization reaction firstly occurred in amorphous region,and then in ordered region and as a result,the ladder cyclized structure was formed.The reaction became violent at 240-280℃ according to DSC test.It was found that with the stabilization for 90 min the stabilization extent was better.