5-Phenylcyclooctene was synthesized and its ring-opening metathesis polymerization were investigated. The monomer was synthesized by selective epoxidation of cis-1,5-cyclocotadiene and its substitution of phenyl Grignard reagent. Polymerizations of 5-phenylcyclocotene provided high polymer in 33% yield with the use of $WCl_6/EtAlCl_2$, but gave oligomer with the use of $WCl_6/Et_2AlCl$ catalyst. The polymer was found to be produced through a ring-opening polymerization by $^1H-NMR$ and IR spectra of monomer and polymer. Glass transition temperature (Tα) of poly(5-pheylcyclocotene) was determined by measurement of the DSC.