The objective of our study was the evaluation of pathogenicity of a local strain of Mamestra brassicae nucleopolyhedrovirus-K1 (MabrNPV-K1) derived from a diseased larva of M. brassicae found in Korea. The effect of temperature and larval instar on the pathogenicity and production of MabrNPV-K1 was determined under laboratory conditions. The median lethal concentration ($LC_{50}$) values of MabrNPV-K1 for 3rd instar larvae were $3.7 imes10^4$ PIBs/larva at $20^{circ}C$, $9.9 imes10^2$ PIBs/larva at $25^{circ}C$ and $3.8 imes10^2$ PIBs/larva at $30^{circ}C$, respectively. The $LC_{50}$ for the 4th instar larvae was similar to that for the 3rd instar larvae. However, the pathogenicity to the 3rd instar larvae was higher than that to the 4th instar larvae. The median lethal time ($LT_{50}$) values of MabrNPV-K1 were 11.4 to 5.0 days at $30^{circ}C$ and 18.3 to 5.5 days at $25^{circ}C$ for the 3rd instar larvae. The $LT_{50}$ value was lowered as temperature went up to $30^{circ}C$ and dependent on viral concentration. In production efficiency of MabrNPV-K1 using M. brassicae larvae, the mortality of the 3rd instar larvae was 100% when inoculated with $1.0 imes10^5$ PIBs/larva and the yield of MabrNPV-K1 was maximal. Regarding the mortality, yield of polyhedra, inoculation doses and required time, the $1.0 imes10^4$PIBs/larva at $30^{circ}C$ was determined as optimal conditions producing polyhedra efficiently.