The armyworm, Spodoptera litura (Lepidoptera: Noctuidae) is the key pest threatening the production of vegetables in the Mekong Delta. Chemicals insecticides have failed to control of this pest as this has developed resistance to almost all synthetic insecticides available. The insect pathogen, Spodoptera litura nucleocapsid nucleopolyherovirus (SpltNPV) has been developed as a commercial biopesticides for control of armyworm. Studies were carried out to determination of LC₅₀ and LT₅₀ values under larboratory conditions by using the first to fifth instars larvae with droplet feeding method. The LC₅₀ values of SpltNPV against 1^st, 2nd and 3rd instar larvae indicated (0.15-8.2) x 10³ OBs/ml; (0.76-11.0) x 10⁴ OBs/ml; (1.1-36.0) x 10⁴ OBs/ml; (0.31-27.0) x 10⁷ OBs/ml and (0.27-5,9) x 10⁸ OBs/ml; respectively. The lethal time required to cause 50% mortality (LT₅₀) showed that for the larvae at the First to fifth instars ranging from 73.25 to 94.25 hours post inoculation (h.p.i); 78.5 to 115.75 h.p.i; 78.74 to 118.51 h.p.i; 159.24 to 175.25 h.p.i and 198.75 to 199.75 h.p.i; respectively. Beside that, the decline in potency and and the efficiency of nine SpltNPV strains virus on the cabbage leaves at 1st, 3th, 5th and 20th days after spray in net house conditions were: 96.4%; 74.9%; 60.0% and 28.7% respectively