In order to examine the requirement for the formation of β-conformation in the course of nascent polymerization step and tile critical length of f-conformation, the peptides containing glycine, L-valine and L-isoleocine were synthesized by heterogeneous polymerization of their α-amino acid N-carboxy anhydrides (NCA) in acetonitrile with n-butylamine as an initiator and the poly-L-alanine were polymerized by those oi active oligtopeptides. The polymerization of L-alanine NCA were carried out by the oligoglycine, oligo-L-valise and oligo-L-isoleucine which were in β-conformation as initiator. The helical α-poly-L-alanine could be formed rather than forming the β-poly-L-alanine, and the critical length of β-oligopeptides were found to be about eight of degree of polymerization.