Cordyceps bassiana is a parasitic fungus and used as a Chinese traditional medicine. It has been called as DongChungHaCho(summer-plant, winter-worm) in China. Acute oral toxicity was examined in male and female ICR mice. Butanol fraction from Cordyceps bassiana(BuCb) was administered orally at a dose of 2,500 mg/kg, 5,000 mg/kg, 10,000 mg/kg. No death and abnormal clinical signs were observed throughout the administration period. The acute toxicity test on mouse did not show any oversign in net body weight gain, food and water consumptions, organ weights, gross pathological findings by different doses of BuCb. Also, biochemical examination revealed no evidence of specific toxicity. These findings show that BuCb has wide margin of safety on acute toxicity with single exposure.