Biomarkers for peptide/protein oxidation under oxidative stress (OS) hold both incredible application potential as well as significant challenges. In this article, liquid chromatography and mass spectrometry were applied to establish a new method for evaluating the oxidation site and degree of peptide oxidized, with its oxidative product serving as biomarker. In the three model peptides, peptide FMRF (containing a methionine) was prone to undergo oxygen addition under UV/H(2)O(2) oxidization, forming a sulfoxide (FM(O)RF) with a stable chromatographic peak separate from the model peptides. The oxidation content of FMRF, expressed as S(FM(O)RF)/(S(FM(O)RF) + S(FMRF)), is positively correlated with oxidation time. Based on sequence analysis of FM(O)RF, the oxidation mechanism (site and extent) of FMRF under UV/H(2)O(2) oxidization was explicitly clarified. By comparing the specific injury to each model peptide, we found that the oxidative products of Met-containing peptides are good biomarkers for OS. This research not only expands the range of biomarkers for OS, but also provides an efficient and accurate method for evaluating oxidation damage to peptides and even proteins.