This study was carried out to understand the activity of pepsin, the proteolytic enzyme, to octapeptide (angiotensin II) in the presence of various synthetic antioxidants as food additives. 1) Dibutyl hydroxy toluene, butyl hydroxyanisole and ethyl protocathechuate did not influence the inhibitory activity of pepsin an the octapeptide as a substrate, but sodium-L-ascorbate inhibited pepsin activity at above 100ppm. However sodium L-ascorbate was completely removed after 30 minutes. 2) Pepsin brought about a quick break up the octapeptide, Asp-Arg-Val-Tyr-Ile-His-Gly-Phe, by splitting the Gly-Phe and Val-Tyr bond. 3) The melting point of synthetized octapeptide was $209-212^{circ}C$, chemical formula and molecula weight were $C_{43}H_{65}N_{13}O_{12}{cdot}CH_3COOH{cdot}H_2O$ and 956.05, respectively. 4) The amino acid mole ratio of synthetized octapeptide by acid hydrolysis were Asp:0.98, Arg: 1.02, Val: 1.00. Tyr: 0.95, Ile: 1.00, His: 1.03, Gly: 0.96, Phe: 1.00.