Abstract: The microstructures of T300 and T700 carbon fibers were investigated by scanning electron microscopy, elemental analysis, X-ray diffraction and Raman spectroscopy. Results showed that the surface of T700 was smooth, whereas ridge-like prominences and grooves were much more obvious on the surface of the T300 fibers, indicating that the former were produced by dry-jet wet spinning and the latter by wet spinning. Grain-like features were seen in the cross section areas of both fibers. T700 fibers had a higher carbon content and less nitrogen than did T300 fibers. The difference in microstructure parameters, such as d002、La and Lc, showed that the heat treatment temperature of T700 during manufacture was higher than that of T300. Compared with T300, the preferred orientation parameter Z of T700 fibers was 2.3° lower, the density 2.27% higher and the microvoid volume 7.54% less. Thus it is the densification that produces the higher tensile strength for T700 fibers.