The cotton leafworm, Spodoptera littoralis (Boisd.) is a serious pest of cotton and other important plants in Egypt. The use of chemical insecticides caused environmental pollution. So that there is strong need to find other safer method than chemical pesticides for insect pest control such as entomopathogenic baculoviruses. The UV effect on Spodoptera littoralis Nuclear Polyhedrosis Virus was studied. The results indicated that exposure of the stock virus concentration (1.1 x 108 Polyhedral Inclusion Bodies per milliliter (PIB /ml)) to Ultraviolet (UV) for 5.0 and 20.0 minutes lead to decrease in the number of PIB/ml, where the PIB/ ml became 2.9x 106 and 6.125 x 105, respectively .These concentrations caused 50 % larval mortality after 18.72 and 21.88 days respectively, while the original non exposed concentration (1.1 x 108) caused 50 % larval mortality after 4.29 days. The results indicated that UV exposure decreased virus concentration, efficacy of virus and Original Activity Remaining. The irradiated virus activity decreased and the corresponding percentage of original activity remaining decreased. The results confirmed that applying the virus most is before sunrise and /or sunset. Also, it is necessary to use natural additives to protect the virus from UV radiation.