Abstract: Two peptides，HG-(α-Asp)-GT和HG-(β-Asp)-GT which contain same amino acids, but α- or β-aspartic acid in the middle of the sequences, respectively, are performed MS/MS studied. The peptide with the α-aspartic acid yields w₃＋H₂O ion in the presence of NaOAc or KOAc. In contrast, the peptide containing β-aspartic acid generates b₂＋H₂O peak in the absence of salts; b₂＋H₂O＋Na and v₃＋Na in the presence of NaOAc and b₂＋H₂O＋K in the presence of KOAc. Thus, the α- and β-aspartic acids in different peptides are differentiated. The ions w₃＋H₂O, v₃＋Na ions are first time identified that can be considered as new feature peaks for the distinction of α- and β-aspartic residues.