Two types of polyacrylonitrile (PAN) copolymers with different coprecursors have been preoxidized under different gradient heating conditions, and the resulting stabilized PAN fibers and carbon fibers examined on a homemade continuous test line. The samples were characterized by differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy, and wide angle X-ray diffraction (WAXD) in order to analyze the effects of copolymer composition on the regularity of the molecular chain of PAN and on the structure transitions during the preoxidation and carbonization stages. The results showed that, due to the greater degree of regularity of the binary copolymer of acrylonitrile (AN) and itaconic acid (IA) ［P(AN/IA)］, the exothermic reaction related to the cyclization reaction occurs at a higher temperature than that for the terpolymer of AN, IA and methyl acrylate (MA) ［P(AN/IA/MA)］. Furthermore, compared with the terpolymer ［P(AN/IA/MA)］, the binary copolymer［P(AN/IA)］had a higher relative cyclization index (RCI) for the preoxidized fibers and a larger crystallite size (Lc) for the final carbon fibers when they were treated under identical preoxidation conditions. In addition, on the basis of the mechanical properties of the carbon fibers it was found that the optimum preoxidization temperature of the binary copolymer ［P(AN/IA)］was higher than that of the terpolymer ［P(AN/IA/MA)］.