Abstract: This paper deals with the morphological character and bioassay result of cytoplasmic polyhcdrosis virus of Thosea postornata. The early third instar larvae were fed with different concentration of the virus kept at 4℃ for 1 year, the LC50 value was 7.04×104 CPB/ml. The LT50 value for the virus (106 CPB / ml) to the third and the fifth instar larvae were 9.6 and 10.5 days respectively. In another word, the LT50 value increased with the increased instar. Silkworms were fed wity fresh Tp-CPV (10s CPB/ml) for 22 days, there were no death and abnormal symptoms.