The objective of this study was to obtain antioxidant peptides from sheep abomasum protein hydrolysates (SAPH). Sheep abomasum proteins were first hydrolyzed using alcalase, neutrase, or papain. Results from three enzyme tests revealed that the hydrolysis process of papain catalyst was most effective as SAPH generated by papain exhibited the highest degree of hydrolysis (DH), yield, and antioxidant activity. The effect of the conditions for hydrolysis with papain was further optimized using the response surface methodology. The highest DH and yield of SAPH (16.86 and 40.65%, respectively) were obtained under the following conditions: hydrolysis temperature, 46 degrees C; hydrolysis time, 3.8 h; and enzyme/substrate ratio, 1.5%. Two novel peptides with high antioxidant activity were purified from SAPH by ultrafiltration, ion exchange chromatography, gel filtration chromatography, and reverse-phase high-performance liquid chromatography. Peptide sequences were determined as Leu-Glu-Asp-Gly-Leu-Lys (LEDGLK, SAPHP-A) and Ile-Asp-Asp-Val-Leu-Lys (IDDVLK, SAPHP-B) with molecular weights of 674.37 and 703.41 Da, respectively. The results of the present study indicated that peptides purified from SAPH were natural antioxidants and could be used as food additives and pharmaceutical products.