Abstract: By controlling the on-line thermal stress of Polyacrylonitrile [P(AN/IA)] at 180 ℃ according to the feature of thermal properties and aggregation structure of PAN binary copolymer[P(AN/IA)], the pre-treated PAN fibers have been continuously stabilized and carbonized to obtain the corresponding stabilized PAN fibers and carbon fibers. The acquired samples were then researched by DSC, FTIR, and WAXD. The analysis results showed that the appropriate thermal stress relaxation treatment of received PAN binary copolymer fibers could improve the tensile strength of resulting carbon fibers. The orientation degree of quasi-crystal areas in PAN fibers has been enhanced with the raise of thermal stress at 180 ℃; however, the activation energy of cyclization reaction for pre-treatment fibers significantly increased and relative cyclization index for pre-oxidized fibers gradually decreased, the interlayer size of corresponding carbon fibers were initially decreased and then increased, while the crystallite size showed the opposite tendency. The mechanical properties of carbon fibers including tensile strength and tensile modulus showed a strong dependence on the thermal stress relaxation treatment of PAN fibers at 180 ℃. Comprehensive study results show that the binary copolymer [P(AN/IA)] with appropriate thermal stress relaxation treatment could transformed into carbon fibers with superior structure parameters and mechanical properities.