Photoinduced cationic copolymerization of tetrahytlroruran (THF) with epichlorohydrin(ECU) in methylene chloride has been carried out in the presence of diphenyliodonium hexafluorophosphate(salt), a photocationic initiator. Random copolymers of THF-ECH were obtained from the monomer mixture system, since the attempt to synthesize THF-ECH diblock copolymer by sequential addition of ECH to the living polymer of poly(THF) was not successful due to the depressed diffusion of ECH, the second component monomer in highly viscous living poly(THF). It has been found that the rate of polymerization of THF is remarkably increased in the presence of trace of ECH, however. the ECH component was not detected from IR spectrum of the copolymer. This implies that ECH plays an important role in the enhancement of initiation, however, not in the propagation. The consumption of ECH in the laten stage of polymerization reveals that ECH takes part in the propagation after most THF is consumed. The living cationic poly(THF) loses its activity in THF-ECH copolymeriLation system due to chain transfer which is taken place when the cationic propagating end is replaced by ECH unit.