Background: Two lipid synthesis inhibitors compounds (spiromesifen SPM and spirotetramat SPT) were used against the second instar larvae of cotton leaf worm, Spodoptera littoralis under laboratory condition. Toxicity of both compounds was tested. Effect of these insecticides on total lipid also was determined. Results: The obtained results showed that both compounds were moderately toxic against the second instar larvae of S. littoralis. The mortality percentages caused by the field rate of spiromesifen and spirotetramat were 51.7 and 60.0%, respectively. The lethal concentrations for 50% of tested insect population (LC 50) were 170.1 and 42.2 ppm, respectively. The total lipid content was determined in treated larvae and compared with the untreated. The percentage of total lipid in spiromesifen-and spirotetramat-treated larvae was 2.17 and 2.21%, respectively, compared with 2.42% in untreated larvae. GC/MS also was used in identification of fatty acids composition of S. littoralis larvae. Conclusion: The results cleared that 13 fatty acids were identified in S. littoralis larvae. The obtained results showed significant difference in percentages of fatty acids with both treated and untreated larvae and also, between tested compounds. Some fatty acids were existed in untreated larvae but not existed in treated larvae.