The cationic ring-opening polymerization of a 1,3-oxazolidine-2-thione derivative (S-L) from L-serine was examined. SL was synthesized by the reaction of L-serine methyl ester hydrochloride with carbon disulfide and hydrogen peroxide in the presence of triethylamine. Cationic polymerizations using methyl trifluoromethanesulfonate (TfOMe), trifluoromethanesulfonic acid (TfOH), and boron trifluoride etherate (BF3.OEt2) as initiators gave polymers with polythiourethane structures in quantitative yields. The obtained polymers showed inverted specific rotations with respect to the monomer, where the large increase of the absolute values suggested the possibility of high-order structures. The molecular weight of the polymer was controlled by the feed ratio of SL to TfOMe and its distribution was constantly narrow (M-w/M-n < 1.14), supporting the controlled character of the polymerization. The reaction mechanism was examined for TfOMe. According to NMR spectroscopic experiments, the polymerization with TfOMe proceeds via an active chain end mechanism in which the protonated cyclic endo iminothiocarbonate 2 is the active species. The propagation step involves a nucleophilic attack of S-L to the active species.