Abstract: Background and Objective: Insecticides are agents used to control the harmful effects of insect pests and vectors on agriculture, livestock and human. The danger associated with most synthetic insecticides currently in use, have made it necessary to look for natural alternatives of which botanicals take an upper hand. The research aimed to determine the mechanism of insecticidal action of oil extracted from the leaf of Cassia occidentalis by carrying out biochemical investigations on insects exposed to the oil. Materials and Methods: Fat bodies and haemolymph were extracted using standard methods from Periplaneta Americana (American cockroach) and Tettigonia virridisima (Bush cricket) exposed for 24 h to 600 mg of oil extracts from the leaves of Cassia occidentalis (Coffee senna). Biological fluids were analyzed for changes in electrolyte composition and other toxicity indices of insects including; acetylcholinesterase, GST and catalase activities. Results: A significant decrease (p<0.05) in k+ and Mg2+ was observed in P. americana as compared to the control, while a significant increase (p<0.05) in GST activity and protein and decrease (p<0.05) in acetyl cholinesterase activity was observed in T. virissdisima as compare to the control. Conclusion: Overall, data from this study provides strong evidence to show that oil extract from the leaf of C. occidentalis exhibits its insecticidal effects through inhibition of acetyl cholinesterase activity of target pests and disruption of ionic composition making it a potential neuromuscular cum organo-chlorine pesticide for future use.