The living nature of cationic polytetrahydrofuran (THF), photoinduced in the presence of diphenyliodonium hexafluorophosphate (initiator), was investigated. In the bulk polymerization of THF, the linear relationship between percent conversion and the number-average molecular weight of the resulting polymer strongly suggests the living nature of this polymer and this was confirmed by the monomer addition technique, that is, cationic poly(THF) is capable of initiating a newly added monomer. The loss of the living nature of the cationic poly(THF) in a polar solvent, dichloromethane, is explained in terms of the stabilization of the five-membered cyclic oxonium ion, a propagating species of cationic polymerization of THF, by ion-pair formation with a less nucleophilic counterion, hexafluorophosphate. Based on the living nature of cationic poly(THF), a diblock copolymer, composed of THF and N-2-(hydroxyethyl) ethyleneimine (HEEI) was synthesized by subsequent monomer addition method; however, it was found that the HEEI block of the compolymer has a nonlinear structure. The factors affecting the structure of the HEEI block are also discussed.