Structure of PAN fibers during pre-oxidation and carbonization was studied using two dimensional small angle X-ray scattering/wide angle X-ray diffraction(2D SAXS/WAXD).The SAXS results show that during pre-oxidation between 180 ℃ and 275 ℃,the volume content of microvoids increases with the temperature increasing,which may be one of reasons for the decrease of tensile strength of pre-oxidized fibers.253 ℃ was the critical transition temperature,the length,diameter,aspect ratio and orientation distribution of microvoids increased with temperature before this temperature and decreased after this temperature.After the high temperature carbonization,lots of spindly microvoids formed.WAXD patterns demonstrate that the crystallite size of PAN fibers first increased before 230 ℃ and then decreased with the increase of temperature during the pre-oxidation.The diffraction peak of PAN fibers at 2θ≈ 17° almost disappeared at the end of preoxidation while the diffraction peak of aromatic structure at 2θ≈ 25° appeared at 253 ℃.During carbonization,the peak intensity at 2θ≈ 25° increased apparently due to the formation of graphite structure.The results obtained give a deep understanding of the microstructure development in the PAN fibers during pre-oxidation and carbonization,which is important for the preparation of high performance carbon fibers. Structure of PAN fibers during pre-oxidation and carbonization was studied using two dimensional small angle X-ray scattering / wide angle X-ray diffraction (2D SAXS / WAXD). The SAXS results show that during pre-oxidation between 180 ° C and 275 ° C , the volume content of microvoids increases with the temperature increasing, which may be one of reasons for the decrease of tensile strength of pre-oxidized fibers. 253 ° C was the critical transition temperature, the length, diameter, aspect ratio and orientation distribution of microvoids increased with temperature before this temperature and decreased after this temperature. After the high temperature carbonization, lots of spindly microvoids formed. WAXD patterns demonstrate that the crystallite size of PAN fibers first increased before 230 ° C and then decreased with the increase of temperature during the pre -oxidation. The diffraction peak of PAN fibers at 2θ≈17 ° almost disappeared at the end of preoxidation while the diffraction peak of aroma tic structure at 2θ≈25 ° at 253 ° C.During carbonization, the peak intensity at 2θ≈25 ° increased apparently due to the formation of graphite structure. The results obtained give a deep understanding of the microstructure development in the PAN fibers during pre -oxidation and carbonization, which is important for the preparation of high performance carbon fibers.