Laboratory, greenhouse experiments were carried out in Giza Research Station during 2006 and field experiments were carried out  at Sids Research Station during 2005/2006 and 2006/2007 to investigate the allelopathic effect of pearl millet and Sudan grass on some winter crops; ( wheat,  barley, flax, faba bean, Egyptian clover and sugar beet) through 2005 and 2006 seasons. Laboratory experiments showed that, pearl millet and Sudan grass had a significant inhibitory effect on seed germination of all winter crops, except Egyptian clover. Seedling growth of flax, berseem and sugar beet was also inhibited by both extract. Greenhouse experiment indicated that all crops were affected by pearl millet and Sudan grass root extracts. Field experiment results indicated that wheat, barley and sugar beet were only the crops affected by root extracts. Three chemical components were detected and identified in pearl millet root extract; (2 benzylthio 2- imidazoline monohydrochlorid) , (pentyl acetate) and  (glucoronic acid   sodium salt hydrate) and another three were detected and identified in Sudan grass root extract (2,6 diemethyl cyclohexane methanol), (4 – Amidinodinopyriodine  menohydrochloride) and  (6 – Alpha-L-arbinozyl – 5 , 7- dihydroxy - 8- beta d – glucopyranozyl flavone).