Synthesis and anionic polymerization of the isocyanate of L-leucine methyl ester (LeuMI) were carried out. LeuMI was prepared by the reaction of L-leucine methyl eater hydrochloride with phosgene. Anionic polymerization of LeuMI was carried out at -50 degrees C in dimethylformamide (DMF) using sodium methoxide, potassium tert-butoxide, methyllithium, or sodium cyanide under a nitrogen atmosphere. The obtained polymers were insoluble in common organic solvents such as DMF, tetrahydrofuran, chloroform, and dimethyl sulfoxide, but were soluble in trifluoroacetic acid. Further, anionic copolymerization of LeuMI with n-butyl isocyanate (BI) was also carried out to observe that the smaller the content of LeuMI unit in the copolymer was, the larger the specific rotation of the polymer. From the circular dichroic spectral analyses of the polymers it was confirmed that the capability of helix formation of poly(LeuMI) was smaller than poly(BI). The relaxation of the helical structure of the polymer in trifluoroacetic acid solution was observed, and the smaller the content of LeuMI unit in the copolymer was, the faster the relaxation of the helical structure.