Brassica napus plant has defensive secondary metabolites against phytophagous insects and pathogens. In extraction process, the end product contains traces of residual solvent, so, the solvent should not be non-toxic and should not interfere with the bioassay. The preliminary biological activity of ethanol, petroleum ether, methylene chloride, ethyl acetate and butanol fractions of B. napus leaves was investigated against the 4th instar larvae of Spodoptera littoralis under laboratory conditions. The ethanolic fraction showed the most toxic effect against the 4th instar larvae of S. littoralis. By phytochemical screening, B. napus leaves contain flavonoids as a major constituent. Four flavonoid compounds were isolated as crystals by column chromatographic technique using silica gel and purified on Sephadex LH-20. Their structures were elucidated as Quercetin (1), Kaempferol (2), Kaempferol-3-O-glucoside (3) and Quercetin-7-O-glucoside (4) by using UV and 1H-NMR techniques. On toxicological studies, LC50 values (0.455 and 0.091 gm/100 ml) at 7 and 14 days post treatment, respectively, showed that the toxicity of B. napus extract increased with increasing concentration and time post treatment. This indicated that B. napus extract can be used as green insecticide to control the cotton leafworm, S. littoralis.