The structure and fragmentation patterns of oligopeptides were studied in this paper, specifically tau (91-97) and tau (401-409), along with some modifications. Such modification is applied to the oligopeptides to prevent some side reactions. Tandem mass spectrometry (MS/MS) is utilized to prove the structures of these tau peptides and identify the product ions formed during the fragmentation. Although there are some limitations, since the sequence of the oligopeptides were not fully determined, the majority sequence of amino acids was, however, still successfully identified. This study aims to provide a deeper understanding of the biological role of tau peptides and its isomers, which would contribute to the prevention and treatment of neurodegenerative diseases, such as Alzheimer's disease.