The toxic effects of some Ruta angustifolia extracts as a bio agent to control Spodoptera littoralis were investigated. Based on LC50 values, the acetonic extract was more effective than hexane and ethyl alcoholic ones. Ergosterol-propyl ester was isolated from the acetonic crud extract by thin layer chromatography and identified using Infra-Red spectrophotometer, Mass Spectrum and evaluated as a fraction for its larvicidal, biological, ultrastructure and biochemical effects on 4th instar S. littoralis larvae. The most prominent biological effects were presented as: prolongation in the total larval duration and a decrease in the percentages of survived larvae. The adult emergence percentages, longevity, fecundity, hatchability were significantly decreased compared to controls. Also, some deformation symptoms were recorded in larvae, pupae and adults. The ultrastructure alterations in S. littoralis 4th instar larvae were observed in cuticle microfilament in muscle myofilaments compared to control. Biochemical responses of the 4th instar S. littoralis larva has a decreas in the activities of both ALT and chitinase and elevation in AST of supernatant S. littoralis larvae compared to control.