The activation effect of various polyhalophenols on the ring-opening polymeriza-tion of cyclopentene with WCl6-i-Bu3Al catalyst has been investigated．Experimental results showed that 2，4，6，-trichlorophenol，2，3，4，6-tetrachlorophenol and pentachlo-rophenol are highly effective activators in the ring-opening polymerization of eyclo-pentene to trans polypentenamer in petroleum ether．In the absence of the activator or in the presence of the phenol the six solvents have the effects on the catalytic activity in the following order：cklorobenzene> toluene (benzene) > petroleum ether (heptane，cyclohexane)．The coeatalytic activity of three diethylaluminium halides is in the following order：Et2AlI>Et2AlCl> Et2AlBr．The trails-vinylene content of polypentenamer is not affected by the nature of polyhalophenols，solvents and alu-minium alkyls，and the polymerization conditions．