Photo-induced polymerization of DL-P-butyrolactone(BL) and its copolymerization with a-methylstyrene(MeSt) were carried out in the presence of diphenyliodonium hexarluorophosphate(salt). a wall known photocationic initiator. It has been fecund that oligo(hytlroty butyrdte) was obtained in high yield, however only oligomers with low molecular weight was results from this system. And an attempt to obtain talc polymer with high molecular weight by copolymerization with MeSt was not successful. This reveals that the reactivity of the cationic species, which is responsible for the cationic propagation of the BL. is so reactive that the propagating cationic species was not stabilized enough to depress the chain transfer reaction by ion pair formation with less nucleophilic hexafluorophosphate ion, supplied from the salt. The reactivity ratio of MeSt (r1) and BL(r2), calculated by Mayo-Lewis method, gave the values of 1.5 and 0.083, respectively.