The amino acid oxidation mechanism has been a research focus in recent years. Although various experimental techniques have been employed to address the problem, it is still a great challenge to identify the oxidation intermediates of amino acids. To explore the potential of theoretical methods in helping elucidating amino acid oxidation mechanisms, one-electron oxidation of a methionine model peptide (N-acetylmethionine amide) was investigated by density functional theory (DFT; including TD-DFT) calculations. The theoretical results not only testified the experimentally identified oxidation mechanisms of the peptide to a large extent but also revealed the contribution of protonated species to the peptide oxidation. All of these findings strongly suggest that DFT methodology has great potential in investigating amino acid oxidation mechanisms.