The antioxidant activity of Isaria (Paecilomyces) sinclairii was determined by measuring its radical scavenging effect on 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radicals. The n- BuOH extract of P. sinclairii showed strong scavenging activity to DPPH. The anti-oxidant potential of the individual fraction was in the order of ethylacetate> n- BuOH>chloroform>n-hexane. The n-BuOH soluble fraction exhibiting strong anti-oxidant activity was further purified by repeated silica gel column chromatography. N-(2-Hydroxyethyl)adenosine (HEA) was isolated as one of the active principles from the n-BuOH layer. The n-BuOH layer, particularly HEA, did not increase the level of nitric oxide (NO) production in vascular endothelial cells that might be related to vasorelaxation such as the action of viagra. In addition, the vascular endothelial growth factor (VEGF) levels showed little or no increase compared with control group with the treatment of I. sinclairii.