Abstract: The stabilization of isotropic pitch-derived fibers (IPFs) and mesophase pitch-derived fibers (MPFs) in air was performed at different heating rates and with different final stabilization temperatures. The stabilized fibers (SFs) and carbon fibers (CFs) were characterized by elemental analysis, FT-IR, TG-MS and SEM to investigate the influence of the degree of oxidation of the SFs on the microstructures and mechanical properties of the CFs. Results showed that a slow heating rate during stabilization was beneficial to the oxidative cross-linking of PFs, and the corresponding CFs had a higher carbonization yield and tensile strength at the lower heating rate. When both IPF and MPF were stabilized at 270 ℃, the two resulting CFs all reached their optimal performance. In addition, the FTIR peak intensity ratio of the C=O band at about 1 700 cm^-1 to the C=C band at 1 600 cm^-1 of the SFs had a good relationship to the carbonization yield and tensile strength of the CFs, and this can be used to optimize the degree of oxidation of the SFs. Moreover, insufficiently stabilized fibers released a great deal of H₂ and CH₄, causing some porosity in the resulting CFs with a low tensile strength, especially for the insufficiently stabilized IPCF. While over-stabilized fibers released a large amount of CO and CO₂, causing cracked textures in the corresponding CFs, especially in the over-stabilized MPCF. Therefore, this research into the stabilization and carbonization behaviors of pitch fibers has great significance for improving the mechanical properties of pitch-based CFs.