The changes of microstructure and Young's modulus of PAN-based carbon fibers during the high temperature treatment (2400-3000 degrees C, stretching 0%) and hot stretching graphitization (0-5%, 2400 degrees C) were compared. It was observed that although the Young's modulus of the fibers could be increased by the two graphitization techniques, the microstructure parameters determined by X-ray diffraction were different for the same value of modulus. The relationship between microstructure and modulus showed that Young's modulus not only depended on the preferred orientation, but also related to the crystallite size (L-c and L-a) and shape (L-a/L-c). On the other hand, it was found that crystallite size of the fibers was mainly affected by heat treatment temperature and the crystallite shape could be altered by hot stretching graphitization. Further investigation indicated that the fibers were composed of turbostratic carbon structure even after heat treated to 3000 degrees C, which could be detected from the absence of 101 and 112 peaks in X-ray diffraction pattern, and the interlayer spacing (d(002)) and preferred orientation (Z) were only 0.3430 nm and 14.71 degrees, respectively.