The field experiment was conducted during 2014-2015 season at the Research Farm of Department of Biology, College of science, Baghdad University to test if the variation in weed population and growth in stands of Barley cultivars was due in part to allelopathic mechanism. The experiment was conducted in a randomized complete block design (RCBD) with four replications. Results revealed that Samer significantly reduced weed density by 54, 42, 42, 40 % and weed dry weight by 61, 68, 58, 45 % after 55, 75, 95 and 115 days from planting, respectively, followed by Rahan and Forat cultivars. Cultivar Arevat showed the lest inhibitory effect to weeds. Subsequent stair-case experiment indicated that root exudates of the Samer cultivar suppressed the test weeds (Malva and wild oat) more than Arevat cultivars giving additional evidence that allelopathic mechanism is involved in weed suppression and the differences in the inhibitory activity between the test cultivars was due to differences in their allelopathic potential. Chemical analysis indicated the presence of phenolics in root exudates of both cultivars (Samer and Arevat) Chemical analysis on these phenolics by HPLC indicated the presence of several allelochemicals, viz, p-hydroxybenzoic, protocatecheuic, vanillic, p-coumaric acid, ferulic acid, sinapic acid and syringic acid with higher concentration in Samer cultivars compare to Arevat cultivar.