A new synthetic method of polypeptide having defined terminal structures was established withusing an acyclic urethane of alpha-amino acid as a monomer. The urethane derivative presented herein is N-(4-nitrophenoxycarbonyl)-gamma-benzyl-L-glutamate, of which polymerization behavior in N,N-dimethylacetamide (DMAc) in the presence of various amines was examined. The polymerization was accelerated remarkably by adding butylamine so that it proceeded smoothly at 30 degrees C to afford the corresponding polycondensate, poly(gamma-benzyl-L-glutamate) (poly(BLG)). Detailed structural analysis of the obtained poly(BLG) with MALDI-TOF mass spectrometry revealed that butylamine was incorporated into the terminal of poly(BLG) and the other terminal was endowed with amino group.