β-Glucans extracted from barley, which mainly contains β-(1,3-1,4)-D-glucan, are used extensively as supplements and food additives due to their wide biologic activities, including a reduction in blood lipid level. In this study, the antioxidant activity of β-glucan was examined to assess potential new benefits associated with β-glucan, because oxidative stress is considered one of the primary causal factors for various diseases and aging. β-Glucan extracted from barley was found to possess significant antioxidant activity. The amount of antioxidant activity was influenced by different physiologic properties (e.g., structure and molecular size) of β-glucan, which varied depending on the source and extraction method used. The antioxidant activity of β-glucan was significantly higher than that of various polymers that are used as food additives. These results indicate that β-glucan has promise as a polymeric excipient for supplement and food additive with antioxidant and other benefits, which may contribute to enhancing health and beauty.