In this paper the process of the thermal stabilization of polyacrylonitrile (PAN) fibres pretreated with cuprous Chloride (CuCl) have been studied in air and nitrogen atmosphere by in-situ infrared spectroscopic method. From the results is was found that the rate of cyclita-tion proceeded furiously at 240-300℃ region and is much faster than the rate of dehydro-genation. It may be inferred that for CuCl treated PAN fibres the aromatization of the structure de veloped extensively and the formed structure more stabilized. It was shown that the incorporation of Cu Cl in the PAN structure enhances the rate of cyclization byabout 1.4 time in air (260℃) and by about 43% in nitrogen (340℃).