Abstract: Objective: To study the differences between antioxidant abilities of polypeptides with different molecular weights extracted from black seed- watermelon seeds.Methods: Multi- stage separation and ultrafiltration techniques were used to treat the enzymatic hydrolysates( SWSPs) of black seed- watermelon seeds protein,and five constituents( SWSPs- L,SWSPs-1,SWSPs-3,SWSPs-5,SWSPs-10) were separated and collected.Then the antioxidant abilities of these constituents were investigated through seven evaluation systems.Results: The antioxidant abilities were obviously different in these five constituents separated from black seed- watermelon seeds.SWSPs-1 with 1~3 ku molecular weights showed the highest superoxide anion radical- scavenging activity while SWSPs- L with molecular weights lower than 1 ku showed multiple antioxidant capacities and it had the similar metal ion chelating activity with EDTA under the same concentration.Conclusions: The antioxidant abilities of polypeptides extracted from black seed- watermelon seeds were closely related to their molecular weight distributions and amino acid compositions,and it was the result of comprehensive effect of all aspects.