This work was carried out to investigate the allelopathic effect of purslane oats nutsedge alfalfa sunflower onion and garlic against some summer (maize grain sorghum broom corn sunflower soybean alfalfa jew''s mallow) and winter (wheat faba bean garlic onion) crops on seedling growth parameters by leachate decayed plants previous plantations and root exudates. In addition trials were done to isolate the bioactive chemical constituents of some allelopathic plants (purslane alfalfa and sunflower).