A study of the effect of temperature on the molecular weight of polyphenyl-acetylene in the thermal polymerization of phenylacetylene for 1．5，6 and 1 0 hours has been undertaken．The results obtained showed．in the temperature range between 150°and 500°，the average molecular weight of the polymer gradually decreased as the tem-perature increased，indicating that degradation of the polymer occurred at higher tempera-ture．The infrared spectra showed that the absorption bands of the polymer obtained at 400。were smaller than that of the polymer obtained at 200°，while the polymer ob-mined at 500°showed no noticeable absorption bands but rather a continuous absorption at an enhanced level(“background effect”)，The ratio of carbon and hydrogen content of the polymers also increased as the temperature of polymerization was raised．These results，in conjunction with the fact that the polymers obtained above 300°showed a dis-tinct fluorescence in solution，are taken to indicate that polyphenylacetylene underwent structural changes at higher temperature and there was，besides degradation to sinail frag-ments，probably also cyclizadon，dehydrogenation，and aromatization，thus converting the chain structure to condensed ring system and thence to three dimensional conjugated sys-tem．The determination of the electron spin resonance of the polymers showed that the concentration of free radicals in the polyphenylacetylene increased in the following order：300°polymer<400°polymer<500°polymer．