Pholopolymerization of tetrahydrofuran(THF) was carried out in the presence of diphenyliodonium hexafluorophosphate(salt) and the mechanism of the polymerization was discussed on the basis of kinetic results. It has been cleared that the cationic polymerization is initiated by the addition of proton, a photolytic product of the salt, to THF forming an cyclic trialkyloxonium ion which is active species, responsible for the cationic propagation. The rate of the polymerization increases with temperature raising, however, decreases with the increasing conversion to polymer because equilibrium between propagation and depropagation is established as the polymer concentration increases. And the hither conversion to polymer and hither molecular weight of resulting polymer at lower temperature are attributable to the shift of equilibrium between propagation and depropagation to polymer side due to the negative enthalpy change of the polymerization. The living nature of the polymerization was confirmed by the monomer addition technique and explained in terms of the stability of the oxonium ion by the formation of ion pair with less nucleophilic hexafluorophospate anion.