Present work aimed to Studies the effect of temperatures on the viability of entomopathogenic H.armigera NPV virus under vitro condition. In this study the nucleopolyhedral virus (NPV), belongs to baculoviridae was isolated from naturally infected larvae of H.armigera. The effect of different temperatures on the pathogenicity of the entomopathogenic virus HaNPV to the American bollworm H. armigera was examined. The viral pathogenicity in terms of larval mortality increased with the increase of temperature; with optimal temperature of 35⁰C for the 2^nd and 4^th instars larvae but the incubation period was shortened with higher incubation temperatures. However, the pathogenicity significantly decreased with the increase of the exposure period. The virus lost its infectivity after exposer to 50⁰ C for ten min. and therefore 50⁰C considered as the thermal inactivation point of HaNPV virus.