It is known that polyphenols in plants have a high antioxidant capacity. However, there is scarce information on its concentration in native plants and the effect of antioxidant capacity to grazing animals. In this study, polyphenol concentration of available plants was investigated in a species-rich grazing area in north-eastern district of Japan. In addition, polyphenol concentration in blood serum of grazing cattle was also measured. Eight beef cows grazed in a grazing area (hill pasture 3.1 ha; forest 16.9 ha) from late spring to mid-autumn (142 days), and four cows among the eight grazed at an orchardgrass pasture in mid-summer (10 days). During the grazing period, foraging behaviour was directly observed, and plant species proportion in ingesta and residence time in the hill pasture and the forest were measured. Based on these results, the top 10–13 species were hand-clipped by mimicking foraging manner of cows, and total polyphenol and catechin concentration were measured. Blood samples of cows were also collected during the grazing period, and potential antioxidant [PAO] and serum total antioxidant status [STAS] were analysed. The cows ingested 17–32 plant species in the hill pasture, and 53–73 species in the forest. In contrast, the cows ingested mainly orchardgrass in the sown pasture in mid-summer. Total polyphenol and catechin concentration were higher in tree leaves (112.8–209.3 g/kg DM, 0.081–6.250 g/kg DM) than monocots (11.9–34.0 g/kg DM, 0–0.159g/kg DM). However, those concentration in ingesta of the cows were low throughout the seasons (35.0–56.9 g/kg DM, 0.108–0.467 g/kg DM), as in the sown pasture (26.2 g/kg DM, 0.158 g/kg DM), due to high proportion of monocots in ingesta at the hill pasture (67–75%). PAO (373.4–455.8 μ mol/L) and STAS (769.8–910.0 μ mol/L) of the cows were almost constant throughout the seasons