To identify the infectivity of four Spodoptera litura nucleopolyhedrovirus (SpltNPV) isolates from diseased larvae of the armyworm: Spodoptera litura (SpltNPV-CT, SpltNPV-AG, SpltNPV-DT, and SpltNPV-KG), collected from the vegetable fields in Can Tho city, An Giang, Dong Thap, and Kien Giang provinces of Viet Nam against S. litura in laboratory conditions. Then, the best SpltNPV isolates will be screened for studies on the development stage of S. litura. The results showed that three isolates of SpltNPV-CT, SpltNPV-AG, and SpltNPV-KG were highly effective on armyworms, over 94% of larvae killed at 9 days after inoculation (DAI) using the droplet feeding method on the 2nd instar larvae. The isolate of SpltNPV-KG had a percent mortality on the 3rd instar of 100% at 7 DAI. Two isolates of SpltNPV-CT and SpltNPV-KG controlled the 4th instar larva of S. litura from 82.1 to 100% at 10 DAI. The maximum yield of occlusion bodies of the fourth Nucleopolyhedrovirus was isolated from 2.87 x 109 to 6.09 x 109 OBs/larva (4th instar larvae). The LD50 value of SpltNPV-KG isolate for the 4th instar larva of S. litura was determined as 1.06 x 108 occlusion bodies/ml.