The uncontrolled use of synthetic insecticides is a great hazard for the environment and consumers. Essential oils were introduced as low toxic agents against mammals and non-targeted insects. In this study, essential oil from aerial parts of blue giant hyssop (Agastache foeniculum [Pursh] Kuntze) (Lamiaceae) was isolated by the water steam distillation method with a Clevenger apparatus, and its chemical composition was studied by gas chromatography mass spectrometry. The toxicity of A. foeniculum essential oil against red flour beetle, Tribolium castaneum (Herbst), and lesser grain borer Rhyzopertha dominica (F.) was evaluated by fumigation at 24, 48, and 72 h exposure times. Estragole and 1,8-cineole were identified as major constituents of the A. foeniculum oil. Fumigation bioassays revealed that A.foeniculum oil had strong insecticidal activity on experimental insects. Rhyzopertha dominica was more susceptible than T. castaneum for all exposure times. Insecticidal activity varied with essential oil concentration and exposure time. Probit analysis showed that increased exposure time and essential oil concentration increased mortality. These results indicated that A. foeniculum essential oil can be applied in the management of stored-product insects to decrease the detrimental effects of synthetic insecticides.