Horse Mint (Mentha longifolia syn. Mentha sylvestris) is a valuable medicinal and aromatic plant belong to Lamiaceae family. In order to evaluate diversity of horse mint in southwest of Iran, 35 ecotypes from six provinces of Lorestan, Fars, Khuzestan, Esfahan, Kohklouye va Boyerahmad and Charmahal va Bakhtiari were examined. To investigate the morphological traits at flowering stage, 10 plants selected in each habitat and 17 qualitative and quantitative morphological traits including plant height, number of branches, branch shoot length, number of flowering shoots, number of branches without flower, the number of inflorescences per plant, inflorescence length, main shoot diameter, number of nodes, leaf length and width, number of leaves per plant, essential oil content, inflorescences color, the state of flowering shoots, status of plant growth flowering date and oil content for each ecotype were investigated. The results showed that there was great diversity among ecotypes. Cluster analysis divided ecotypes into four groups. Also Factor analysis showed that five major components which 74.23 percent of the total of variance data to justify. Number of branches, branch shoot length, number of flowering shoots and inflorescences were the first component. Plant height had a significantly positive correlation with inflorescence length, main shoot diameter, number of nodes, leaf length and width, number of leaves per plant. The results of this study showed that the two ecotypes of 6 and 13 (Lorestan and Fars) due to good characteristics and the large amount of oil are a good potential for domestication of this species.

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