This study was undertaken to investigate the toxicity of four natural source essential oils (Trigonella foenum graecu,  Sesamum indica,  Eucalyptus camaldulensis and Nigella sativa) on 4^th larval instar of  Spodoptera littoralis. Biochemical changes in larvae induced by treatment with LC₅₀ of tested essential oils were also studied. Bioassay technique was performed using thin film to estimate LC₂₅, LC₅₀ and LC₉₀ values. Results showed that, T. foenum graecu was the most effective oil followed by S. indica, E. camaldulensis and finally N. sativa. As regards to biochemical changes induced by treatment with LC₅₀ of each oil. Remarkable reduction in acetyl cholinesterase (AChE) enzyme was recorded in T. foenum graecu and N. Sativa treatments. However E. camaldulensis and S. indica induced significant elevation in glutamate oxaloacetate transaminase (GOT) enzyme, while glutamic pyruvic transaminase (GPT) reported significant elevation in all treatments. Remarkable reduction in total protein content was observed in N.sativa treatment. Whereas, treatment with T. foenum graecum oil induced significant increase in total protein content. In conclusion, results indicated that the plant essential oils had insecticidal effect and also showed adverse effect on the tested biochemical parameters, suggesting the possibility of using them as an alternative to conventional insecticides for cotton leafworm control.