Abstract: X-ray wide angle diffraction/small angle scattering, Raman spectroscopy and high resolution transmission electron microscopy were used to characterize the crystalline structure, pore structure, radial structural heterogeneity, degree of graphitization, internal residual stress, crystalline orientation and fractal phenomena of various grades (T and MJ) of polyacrylonitrile-based carbon fibers made by Toray Inc, Japan. Results showed that compared with the T series fibers, the MJ series fibers had a significantly lower internal residual stress, better structural orientation and a much higher degree of graphitization, but the dimensions of the microvoids and the radial inhomogeneity were increased, which revealed the significant influence of graphitization conditions in making MJ series fibers on the microstructures of carbon fibers.