Oxidation of key amino acid residues in bioactive peptides and proteins by endogenous oxidants can leads to life disorders.It is well known that in vivo oxidation of methionine residue in proteins to methionine sulfoxide residue by reactive oxygen species (ROS) plays a critical role in aging and degenerative diseases.The combination of tandem mass spectrometry (MS/MS) with high-performance liquid chromatography is becoming attractive in characterizing structures of peptides and proteins compared with traditional analysis methods, since it can provide useful information to elucidate the structural changes including the modification site through fragments.