Objective To observe the antifatigue effect of Cordyceps sinensis (Sacc), and to investigate its mechanism. Methods The mice were divided randomly into three groups: control group, low dose Sacc group and high dose Sacc group. In Sacc group mice were fed with Sacc, in control group mice were fed with the same amount of physiological saline.14 days later, the swiming time of loading mice and hypoxia bearing time under normal atmospheric pressure were observed,and the concentration of lactic acid in serum, the contents of myoglobin (Mb) and malondialdehyde(MDA),and the activity of lactate dehydrogenase(LDH) were determined.Results Compared with control group,both the swimming time of loading mice and hypoxia bearing time prolonged significantly ( P <0.01); activity of LDH increased markedly ( P <0.05 or 0.01); contents of Mb and MDA decreased distinctly ( P <0.01). Conclusion Sacc can increase moving capability of mice,and has antifatique effect.The mechanism may be that Sacc can increase storage of energy and oxygen, and has antioxygenative effect.