To evaluate the immune activation and reactive oxygen species scavenging activity of Cordyceps militaris polysaccharides (CMP) in vivo, 90 male BALB/c mice were randomly divided into six groups. The mice in the three experimental groups were given cyclophosphamide at 80 mg/kg/d via intraperitoneal injection and , 35, or 70 mg/kg body weight CMP via gavage. The lymphocyte proliferation, phagocytic index, and biochemical parameters were measured. The results show that the administration of CMP was able to overcome the CY-induced immunosuppression, significantly increased the spleen and thymus indices, and enhanced the spleen lymphocyte activity and macrophage function. CMP can also improve the antioxidation activity in immunosuppressed mice, significantly.