Monoterpenoids and their derivatives from plant essential oils show potential to be a good alternative to conventional insecticides due to their relatively high toxicity to insect pests, low toxicity to non-target organisms, and biodegradability in the environment. This dissertation has investigated the possible mechanisms of action of monoterpenoid insecticides in the insect nervous system, including an inhibitory neurotransmitter receptor, the GABA receptor, and an excitatory neurotransmitter, the nicotinic acetylcholine receptor (nAChR). By using radioligand binding assays, 36Cl- uptake assays, and quantitative structure-activity relationship assays, monoterpenoid insecticides were suggested to bind to these two receptors, and could interfere with the function of them in the nervous system, which might be the reason why these naturally occurring compounds have toxicity to insects. In the Chapter 2, carvacrol, pulegone, and thymol showed binding activities on insect GABA receptor, and inhibitory effects on the insect nervous system. In the Chapter 3, some structural and chemical properties of monoterpenoids, including Mulliken charges, log P values, and total energy levels, were indicated to be related to the binding between monoterpenoids and the insect GABA receptor. In the Chapter 4, a potent monoterpenoid insecticide, carvacrol, showed binding activity on insect nAChR at a new binding site. This dissertation will help us to understand the mechanism of action of monoterpenoid insecticides further, and is meaningful to discover and develop novel green insecticides.