Essential oil extracted from plants has been widely investigated for pest control properties, with some proving to be toxic in insect pests. In this study, effect of the synergist diethyl maleate (DEM) on toxicity of one monoterpenoids, linalool was studied against two most common stored-product insects such as Callosobruchus maculatus and Rhyzopertha dominica. Diethyl maleate was combined in mass ratios (1:8 and 1:4) with acetone used and applied on C. maculatus and R. dominica adult. Five concentrations of linalool were tested with four replications at 24 and 48 h with 30 adult insect in each replication. After 24 h of exposure, the LC 50 values were estimated to be 23.61 and 31.01 µl/l air, and after 48 h, they were 15.07 and 21.84 for each insect, respectively. A combination of inalool with the synergist after 24 h of exposure, the LC 50 values was estimated to be 11.93 and 13.07 µl/l air and after 48 h, they were 7.38 and 7.93, respectively for each insect. The synergist is able to block the specific system of enzymes involved in selection of tolerance in susceptible generations. Diethyl maleate is an inhibitor of glutathione S-transferase (GST) activity. These results show that diethyl maleate decrease doses of linalool.