The acrylate comonomer in polyacrylonitrile(PAN) precursor markedly influences the microstructure of PAN fiber and its resulting carbon fiber. In this study, the structural change by the heat treatment of PAN copolymers(acrylonitrile-co-methyl acrylate) in the presence of air up to 26$0^{circ}C$ has been examined by rosing both physical and chemical techniques. It has been found that the monomer compositions of 98 : 2 to 97 : 3 (AN : MA mole%) are suitable for the precursor preparation. The cyclization mechanism and fine structural are significantly affected if the MA composition is more than 3%. With increasing MA content, the exothermic onset and peak in the thermogram was found to shift toward higher temperatures, while the aromaticity index decreased. These results indicate that MA does not activate but interfere the cyclization. Additionally, the tacticities of copolymers are found to be nearly identical regardless of the copolymer compositions, and the cyclization occurred nonstereospecifically during the initial heat treatment at 24$0^{circ}C$ for 1hr.