Cordyceps is reputed for its broad biological activities and as a tonic for replenishing vital function in Chinese traditional medicines. As an attempt to obtain fundamental data for the development of new type Cordyceps, the effects of the fruiting bodies of cultivated fungus of Paecilomyces japonica grown on silkworm larvae on fatigue and hematotoxicity were investigated. The antifatigue activity of dongchongxiacao (Paecilomyces japonica) was studied in mice using weight loaded forced swimming performance method. The water extract of dongchongxiacao(25 mg/kg of body weight, intraperitoneal injection at 12 and 36 hours before irradiation and 30 minute and 24 hours after irradiation) showed a significant antifatigue effect in normal mice and radiation-induced fatigue mice (p<0.0005). In the experiment on the improvement of hematotoxicity, a significant difference from the irradiated controls was seen at day 3 of the group treated with dongchongxiacao and substantial differences from the irradiated controls at day 7 was also detected.