Abstract: The stabilization, carbonization and graphitization processes in the preparation of isotropic and anisotropic pitch fibers were studied by elemental analysis, TG, DSC, FT-IR, XRD and STM. The reaction mechanisms were deduced and kinetics parameters evaluated. The change of mechanical properties and crystalline parameters during graphitization were also obtained. Results show that the optimum oxygen content for isotropic pitch fibers is 19.63 w/%, but that for anisotropic pitch fibers is 8.03 w/%. The d002, Lc, La of isotropic pitch based graphite fibers (IPGF) are 0.3432, 3.432 and 8.053 nm respectively, while for anisotropic pitch based graphite fibers (APGF) they are 0.3391, 25.69 and 53.05nm. Tensile strength and modulus are respectively 596MPa and 58 GPa for IPGF, and 2956 MPa and 632 GPa for APGF.