Abstract: Polyacrylonitrile fibers were pre-oxidized in air at temperatures from 180℃ to 265℃. The morphological rearrangement, chemical reactions and the variation of thermal properties during the continuous thermal oxidative stabilization were characterized by X-ray diffraction, FT-IR, DSC and TG. Results showed that: (1) When the pre-oxidative temperature was below 200℃, the cyclization reaction was slow. When the temperature increased from 200℃ to 230℃, the cyclization index increased rapidly and the reaction was drastic. The reaction rate reduced after the pre-oxidative temperature reached above 230℃. (2) The pre-oxidation reactions were first initiated in an amorphous region, thereafter the reactions propagated to the crystalline regime at high temperatures where reactions were drastic. (3) When the pre-oxidative temperature was below 210℃, the crystal size increased, corresponding to stress crystallization. The crystallite size decreased rapidly after the pre-oxidative temperature reached above 210℃.