In order to investigate the effect of some medicinal plants on weed density and diversity, an experiment was conducted based on a complete randomized block design with 3 replications at the agricultural research station, Ferdowsi University of Mashhad, Iran, during 2012 and 2013. Treatments included 13 medicinal species (Origanum vulgar, Hyssopus officinalis, Tanacetum parthenium, Echinaceae purpurea, Ruta graveolens, Althaea officinalis, Hypericum perforatum, Trigonella foenum-graecum, Oenothera erythrosepala, Malva sylvestris, Securigera securidaca, Achillea millefolium, Digitalis purpurea). The results indicated that the effect of medicinal species on total weed density and weight was significant. The highest total weed density was observed in Digitalis purpurea. The lowest total weed density was obtained in Tanacetum parthenium and Trigonella foenum-graecum. Also the highest and the lowest total dry weight of weeds were observed in Digitalis purpurea and Trigonella foenum-graecum, respectively. The most important weeds in Digitalis purpurea and Trigonella foenum-graecum plots were Convolvulus arvensis and Solanum nigrum. Shannon and Margalof indices of weeds were affected by plant species, significantly. The highest Shannon and Margalof indices were observed in Malva sylvestris and Digitalis purpurea.

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