Medicinal plant may be used as a viable weed control technology in organic systems. This study was carried out to evaluate the allelopathic effect of foliage extract of Achillea wilhelmsii on seed germination and growth characteristic of soybean and Amaranthus retroflexus. In two series of laboratory and greenhouse experiment in Jihad-e-agriculture research center of Sarayan in 2010. Plants aerial parts in blooming were collected from around ferdows at and aerial part of Achillea wilhelmsii was extracted using the soxhlet extractor. In laboratory experiment were conducted based on completely randomized design with three replications, the effect of different concentration aquatic extract of santal in yarrow (12.5, 25, 37.5 g/l) with distilled water as control as on germination seed Amaranthus retroflexus and Glycins max, were investigated. In the greenhouse, which is based on randomized completed block design in addition to the concentration of the extract adding four levels (0 as control and 5, 10, 15%) of decomposed residues of Achillea wilhelmsii in with three replications was aded to pots of Amaranthus retroflexus and Glycins max. According to the laboratory results, concentrations of extract were completely inhibited germination of A. retroflexus. However significant reduction of growth characteristic of soybean was observed at high level of extracts. Height, biomass and seed yield of redroot pigweed decreased 80%, 40% and 40% in the concentration of 37.5 g/l of santal in yarrow respectively. Decomposed residues were strongly reduced the growth and yield characteristic of redroot pigweed with a little effect on growth of soybean. They decreased seed yield 75%. The results generally showed that Achillea wilhelmsii had allelopathic ability and can be used to control of Amaranthus retroflexus in soybean.