Allelopathy may be used as a sustainable approch for weed control in the field. Fennel is a medicinal plant with allelochemical properties wich has a long history of production both in Iran and in the world. In order to investigate the allelopathic action of fennel, three experiments were conducted in Department of Agronomy and Plant Breeding of Isfahan University of Technology, Isfahan Iran in 2012-2013. In the first experiment, aquous extract of fennel was used to examine the allelochomical potential of 10 fennel genotypes obtained from different regions of Ira (Isfahan , Avisina, Kerman, Shiraz, Mashhad, Yazd, Ardabil, Uromiyeh, Hamedan and Bushehr) on two noxios weeds ( Amaranthus retroflexus and Portulaca oleraceus) and sesame oil seed crop. Aquous extracts were prepared from aerial parts of fennel in concentrations of 0, 6.3, 12.5, 25, 50 and 100%. Seedling attributes of target species including radicle and plumule lengthes, radicale and plumule fresh weights, plumule dry weight, and germination percent and velocity were measured. Results revealed that the target species response to allelochemical effect of fennel varied with fennel genotype. Genotypes Mashhad and Kerman were proven to be the most allelopathic genotypes, and genotype Hamedan and Ardabil were found to leave minimal allelopathic effects on the target species. In the second experiment, allelopathic potential of five fennel genotypes (Isfahan, Avisina, Mashhad, Ardabil and Bushehr) on the basis of the effect of their essential oil on pigweed and purslane weedy species were studied. Essential oils of fennel in concentrations of 0, 100, 200, 400, 600 and 800 ppm were prepared from leaves and their effects on germination and growth of the two weedy species seedlings were investigated. Seedling attributes of the target species including radicle and plumule lengthes, radicle and plumule fresh weights, plumule dry weight and germination percent were measured. With increasing in essential oil concentration of the medium root and shoot length, root and shoot fresh weight, shoot dry weight and germination percentage of target species were decreased. Maximum inhibitpry activity was observed at essential oil concentrations of 600 and 800 ppm. Bushehr genotype had the greatest allelopathic effect on root and shoot lengths, root and shoot fresh weights, shoot dry weight and germination percent of target species, and was therefore introduced, based on its essential oil, as a genotype with strong allelopathic effect. Yazd and Isfahan were identified as genotypes with week allelopathic effects. The two weedy species did not differ significantly in terms of their sensitivity to allelopathic effect of this medicinal plant. Active ingredients analysis of fennel essential oil revealed that genotypes with high levels of "? - pinene " possess high capacities of allelopathic effect, while genotypes with high levels of "fenchone" were proven to leave minimal allelopathic effects. The third experiment was designed to investigate the suppressing effect of fennel genotypes on the weed growth under natural field conditions. At the end of the second growing season of fennel, sampling was conducted in October 2012. Random samplings in two locations of each plot were performed in 3 replications. Weeds were colleted both in the vicinity of the main stem of fennel crop (to identify both light competion and allelopathic effects of fennel on weeds) and also farther from the crop main stem (where the light reachs to the surface and it does not leave confounding effects on allelopathic effects of fennel on the weeds). Naturally occuring weed species included pigweed, purslane, lambsquarters and field bindweed. Shoot height, shoot fresh and dry weights of weeds were measured. The results of this experiment revealed that field bindweed and lamsquarter had the greatest freqency, while pigweed and purslane had the smallest frequency. Genotypes Hamedan and Kerman indicated the smallest interfering effects while genotypes Busher, Avisina, Ardabil, Shiraz and Uromiyeh were found to have the greatest interfering effects on the weeds. Genotypes Hamedan , Uromiyeh, Avisina and Ardabil had the weakest allelophathic effects while genotypes Shiraz and Yazd were found to exert the strongest allelophathic effect on the weeds under field conditions. Across all experiments conducted here and averaged over the fennel genotypes examined, the greatest allelopathic effect of this mediciual plant was concluded to be on the weedy pigweed species. Key words : Allelopathy, aquous extract, essential oil, fennel genotypes, growth inhibition.