Abstract: Empedobacterbrevis has been widely applied as a microbial agentto control the infestation of Ectropisobliqua Prout (Lepidoptera: Geometridae) on tea bushes at plantations. Thisstudy aimed to decipher the mechanisms of the insecticidal function of E. brevis. Changes on the cellularstructure of the midgut and hemolymph enzyme activity in E. obliqua after being treated with E. brevis were examined for the analysis. A toxicity assay was usedto determine the lethal concentration (LC₅₀) of the microbial agenton the 5^th instar larvae of E.obliqua. The effect on the midgut morphology was observed undertransmission electron microscopy (TEM).  Finally,the hemolymph enzyme activity were detected by themicroplate reader. The results showedthat the LC₅₀ was 3.077×10⁵ spores/mL, the midgut cell and organellestructures were disrupted, and the hemolymph enzymes significantly affected. The activities of the detoxifying enzymes in 5^th instar larvae of E. obliqua,including AchE, CarE and GSTs were increased in 12h, then declines in 24 h. It appeared that the death of E. obliqua caused by E. brevis might be related to thealtered hemolymph detoxification enzyme activity and the destructed midgut cellstructure of the insect.