Objectives: To clarify the antioxidative effects of kakisu (persimmon vinegar), plasma antioxidant power and urinary oxidative stress level in healthy subjects were measured using enzyme immunological assays. Methods: Eighty-one subjects (age 30-69, 58.4±0.8) were randomly divided into two groups using a crossover design. Group A drank kakisu for 56 days starting in March, whereas group B drank kakisu for 54 days starting in June. Copper reducing equivalent level in plasma was measured as antioxidant power, and urinary 8-isoprostane (8-iso-prostaglandin F_2α) level was measured as oxidative stress marker. Results: Baseline plasma antioxidant power and urinary 8-isoprostane level showed no significant correlation among the subjects in this study. By drinking kakisu for 8 weeks, total antioxidant power significantly increased, and urinary 8-isoprostane level decreased. Total antioxidant power increased more markedly in group A than in group B. In contrast, urinary oxidative stress level decreased more markedly in group B than in group A. Smoking habits significantly correlated with urinary 8-isoprostane level. Males were more sensitive to the antioxidative effects of kakisu than females. Conclusions: Kakisu has antioxidative effects that increase plasma antioxidant power and reduce urinary 8-isoprostane level. Further study is needed to clarify the influence of season and gender on such antioxidative effects.