Oligo-peptides have been focused on by chemists recently because of the strong bioactivity an activity as drug or precursor of drug. α-Amino acid could self-assemble into oligo-peptides with the assistance of inorganic phosphorus reagents, so it could provide a new method to synthesize peptides. In this paper, the self-assembly reaction of L-Phenylalanine mediated by phosphorus oxychloride were studied by using ESI-MS. The proper condition of self-assembly reaction of L-Phe was reported. The reaction products were purified by RP-HPLC and Phe-Phe dipeptide was characterized by the means of IR, 1H-NMR, 13C-NMR and ESI-MS/MS.