Chlorpyrifos and Imidacloprid are belongs to two main groups of insecticides that are widely used in insect control. Many researches had been discussed the effect of both groups on non-target organisms and the environment. So, it necessary to reduce the used doses of this groups to save the environment. The effect of pre-treatment with lufenuron (chitin synthesis inhibitor) at sub lethal dose (LD₂₅) on the efficacy of Chlorpyrifos and Imidacloprid on Spodoptera littoralis was studied using topical application method. The toxicity of both Chlorpyrifos and Imidacloprid increased after 24hrs of pre-treatment with LD₂₅ of Lufenuron by 55 and 250 times for  Chlorpyrifos and Imidacloprid, respectively  compared with the toxicity of each compound  alone. Histological studies showed that the treatment of 2^nd instar larvae with LD₂₅ of Lufenuron  caused obvious separation of the hypodermis layer from the endocuticle, abnormalities in the epicuticle morphology , rupture in muscles, irregular and great disintegration  of cuticle and hypodermis. In-vivo interaction of LD₂₅ values of Chlorpyrifos, Imidacloprid and Lufenuron with Alkaline and Acid phosphatase caused significant increase in the activity of Alkaline phosphatase while Acid phosphatase activity was significantly decreased in all treatment.