The structural properties of carbon fibers with different tensile modulus have been investigated. Carbon fibers used in this study are: 1- high ductility impregnated pitch based carbon fibers with the tensile modulus 110 and 155 GPa, 2- standard tensile modulus (high tenacity) polyacrylonitrile-based carbon fibers with the tensile modulus 240 GPa and 265 GPa, 3- intermediate tensile modulus (290 GPa), 4- high tensile modulus (390 GPa; 425GPa) and 5- ultrahigh tensile modulus (520 GPa; 630 GPa;720 GPa; 880 GPa; 923 GPa). Utilizing the Raman spectroscopy, the characteristic ID/IG ratio and the existence of the G’-band have been analyzed. Via the wide-angle x-Ray diffraction patterns, the crystal parameters, crystal thickness (Lc, La||, La), the crystal aspect ratio (La/Lc) and the apparent crystallinity, the graphitizing degree and the density have been characterized. The orientation factor of the crystals has been calculated for the different fibers from the azimuthal distributions of the respective (002) reflex. Conclusions regarding correlation of the Raman spectroscopy characteristics to the aspect ratio (La/Lc) and the orientation degree of the crystals have been drawn. Additionally, the nature of the pore structure of the carbon and graphite fibers has been studied utilizing the ESAX, SAX and MSAX techniques. The pore size and shape have been correlated to the Raman aspect ratio ID/IG.