N,O-Bis(trimethylsilyl)-alpha-amino acids (N,O-BTMS-AA), mediated by o-phenylene phosphorochloridate (PPC), could oligomerize into polypeptides. The mechanism might go through sequential steps, i.e., the activation of amino acid, the elongation of peptide chain, and the termination of elongation reaction, as can be traced by P-31 NMR spectroscopy. The activated amino acid was a five-membered cyclic pentacoordinate phosphoric-carboxylic mixed anhydride. The nucleophilic attack of the amino group of an amino acid or a peptide on the carbonyl group of the intermediate led to the formation of peptide with release of a phosphate ester. The repetition of the reaction sequence generated successively longer N, O-bis(trimethylsilyl)peptides, which were then hydrolyzed to give a series of oligopeptides. It is worth noting that only the N,O-bis(trimethylsilyl)-alpha-amino acids, not the N, O-bis(trimethylsilyl)-beta-amino acids, could be activated and assemble into polypeptides. The mechanism of the five-membered cyclic pentacoordiate phosphoric-amino acid anhydride intermediate showed that phosphorus could choose alpha-amino acids in the prebiotic synthesis of polypeptides and biosynthesis of proteins.