Two dimensional small angle X-ray scattering (2D-SAXS) method was used to study the characteristic long period structure of polyacrylonitrile precursor. Based on the high resolution measurements

the scattering signal comes from the micro-voids and long period structure was separated successfully by the mathematic analog computation treatment. Research method for the long period structure which includes the arrangement of the crystalline and amorphous regions for PAN precursor was established. Combining with the correlation function

analysis the detail structural parameters such as the long period

the thickness of crystalline and amorphous regions for PAN fiber were obtained. The crytallinity of the fiber was calculated and compared to the data which was determined by traditional WAXD measurement.