Abstract: Aim ：To preliminarily investigate the antioxidant activity of the acidic exopolysaccharide（EPS-A） from cultivated Cordyceps sisnensis in vitro and in vivo. Methods ：PC12 cells were injured by H₂O₂ after the cells were treated with different doses of EPS-A for 6 h，and then the levels of malondialdelyde(MDA) and superoxide dismutase (SOD) in PC12 cells were determined.H22 cells were inoculated into the right oxter of each mouse after ICR mice were treated with EPS-A for 7 days，and tumor-bearing mice were treated with EPS-A for 7 days.MDA level and SOD activity in liver，brain and serum were assayed，and tumor weight of each mouse was measured as well. Results ：EPS-A significantly decreased the production of MDA and increased SOD activity in H₂O₂2-injured PC12 cells.Moreover，EPS-A inhibited H22 tumor growth in mice，in which the level of MDA in liver and brain was reduced，and SOD activity in liver，brain and serum was enhanced. Conclusion ：EPS-A exhibited antioxidant activity in cell cultures and mice.