Abstract: The changes of preferred orientation of PAN-based carbon fibers during continuous high temperature graphitization and hot stretching graphitization were investigated by X-ray diffraction (XRD). The results showed that the degree of preferred orientation within PAN-based carbon fibers increased with the increasing heat treatment temperature (HTT) and the ratio of hot stretching，respectively. Both in high temperature graphitization and in hot stretching graphitization the fiber’s tensile modulus increased with increasing preferred orientation，but the values of preferred orientation (Z) determined by XRD were different for the same value of modulus. The relationship between microstructure and modulus showed that tensile modulus did not only depend on the preferred orientation，but also related to the crystallite size. In addition，even after heat treated to 3 000 ℃，the fiber’s preferred orientation (Z) was only 14.71°，indicating the fiber’s turbostratic layers were still not highly aligned.