The radiation responses of prodenia litura nuclear polyhedrosis viruses（NPV） to gamma-ray and electron-beam have been investigated by measuring survival fraction and electron spin resonances（ESR） of irradiated samples. lonization radiation can produce long lived free radicals in NPV and its concentration is proportional to dose of radiation. The curves of survival fraction with either dose of radiation or concentration of free radicals were shouldered. Radiation damage mechanism of NPV have been studied by the correlation between survival fraction

free radical concentration and dose of radiation. It is posible to use ESR method for determing infectivity of NPV instead of biology assay.