In order to study theability of hull extracts of Iranian rice cultivars on germination and seedling growth of barnyard grass, two laboratory and greenhouse experiments were conducted ina completely randomized block design with factorial arrangement and three replications. This study wasundertaken to evaluate the allelopathic potential aqueous extract of 20 hull rice (Oryza sativa) cultivars on growth factor of barnyard grass (Echinochloa cruss-galli). This research was performed in 2007 at the Department of Agronomy of Tarbiat Modares University. Aqueous extract of hull rice in four concentrations (control, 5, 10 and 15%) were used for germination test and seedling growth of barnyard grass seeds. In the laboratory, we found that a few varieties product and released allelochemical for inhibiting and more cultivars stimulation growth of barnyard grass. Khazar variety showed the highest inhibition on germination percentage, germination rate, shoot length, root length, root dry weight and shoot dry weight. In addition, the breeding and domestic varieties were inhibitor and simulator of barnyard grass respectively. In the field experiment, Khazar cultivar reduced germination percentage, germination rate, root length, root dry weight, total dry weight and shoot dry weight of barnyard grass and Neda cultivar showed the greatest inhibition on shoot length. This study show that there are differences among the varieties of rice for allelopathic inhibition of barnyard grass seeds and genetic and morphological rice characteristics could be used as selection markets for allelopathic rice varieties and maybe used as a natural herbicide to reduce the population of barnyard grass in field. Increasing concentration of aqueous extract increased effects of inhibition and stimulation of various cultivars on barnyard grass. Survey morphological and genetic traits of rice varieties showed an existing relationship allelopathy potential of rice on barnyard grass.