The benzoyl peroxide-induced polymerization of phenylacetylene has been studied．At 60°．when the amount of the initiator used was less than 1 mole per cent of the monomer，very little polymerization occurred；when 2—4 mole per cent of the initiator was used，polymerization took place readily． When the temperature of polymerization was raised from 60°to 130°，the rate of polymerization increased markedly．The average molecular weight of the polyphenylacetylenes obtained was 600—1500．The in-frared spectrum of the polymer showed characteristic absorption for the C6H5COO-ter-minal group．In the infrared spectrum of the polymer obtained at 160°，the charac-teristic absorption peaks for the C=O and C—O bonds essentially vanished．The thermal polymerization of phenylacetylene occurred to a negligible extent be-low 100°；at 100—160°，the rate of polymerization、increased with increasing tempera-ture．Boron trifluoride could induce the rapid polymerization of phenylacetylene whether at room temperature or lower．The average molecular weight of the polyphenylacetylene obtained was over 2000．The infrared spectra of the polyphenylacetylenes showed great similarities，except the characteristic absorption for the C6H5COO-grouping in the benzoyl peroxide-initiated polymerization product．In comparison with the infrared spectrum of polyphenylace-tylene reported bv Okamoto et al[6] there was a minor difference in the 770—700 cm-1 region，while most parts of the spectra are very much similar；this indicates that the poly-phenylacetylenes obtained by the various methods reported in this work have probably the same chain structure.