The formation and evolution of structure, and the changes of properties during the preoxidation, precarbonization, and carbonization of different PAN precursors were studied by the combination of DSC, FT-IR, SEM and some traditional measurements, such as density and mechanical properties of various fibers. The exothermic regime of polyacrylonitrile-based precursors made of acrylonitrile / itaconic acid (AN / IA) copolymers or acrylonitrile / acrylamide (AN /AM) copolymers is much broader and the cyclization reaction starts at lower temperature, compared to that of PAN homopolymer precursors, but AM appears to be more effective in separating the exothermic reactions corresponding to preoxidation stages in DSC curves as compared to IA. If AN / IA (97.5 / 2.5 w / w) precursors and AN /AM (97.5 / 2.5 w / w) precursors are designated as P1 and P2, respectively, the AM-containing commercial precursors (P3) are thermally more stable than the P2 ones, and the density of P3 is higher than that of P1 or P2. This may result from the difference of aggregation morphology among the original precursors, since it is dense for P3 precursors, whereas P2 and P1 precursors have some voids. The tensile strength of resultant carbon fibers from P3 precursors was better than that of carbon fibers from P2 or P1 after identical conditions of preoxidation are employed.