Abstract: Phenolic substances, lipoidal matters and fatty acid contents of some cruciferous powdered seeds, Raphanus sativus L., Brassica oleracea L. var capitata, Sinapis alba L., Brassica nigra Koch, Eruca sativa Mill, Brassica napus L. and Lipidium sativum L., were determined. The results indicated that R. sativus L., B. napus L. and B. oleracea L. var capitata contained higher percentage of phenolic compounds, lipoidal matters and unsaturated fatty acids, respectively, compared to other tested plants. The aqueous and ethanolic extracts of the above seeds were prepared. The alcoholic extract was fractionated with petroleum ether 60-80oC, chloroform and ethyl acetate, respectively. The prepared extracts and fractions were subjected to phytochemical screening and the results indicated the presence of various active constituents. Their allelopathic effect on Rhizoctonia solani Kuhn was tested in vitro. The tested extracts have variable inhibitory effect on R. solani Kuhn. The effect depends on the concentration of the extracts, plant species and the active constituents. Rhizoctania solani infested and non-infested soil sowed with cotton (Gossypium barbadense L ) seeds were treated with the above powdered seeds. All treatments reduced cotton seed germination, decreased significantly root length but increased seedling shoot height. The results indicated that application of powdered seeds of R. sativus L., E. sativa MILL and S. alba L., to the soil infested with R. solani Kuhn, enhanced the germination percentage of cotton, reduced damping off precentage and improved the growth criteria of the cotton seedlings. Fatty acids composition and mineral contents of the untreated cotton and infested cotton seedlings treated with seed powders were also investigated