Abstract: ：The seedlings of transgenic Medicago sativa overexpressing BADH were stressed by different contents of PEG6000. Some physiological and biological changes were tested after14 days treatment. The content of Malonaldehyde (MDA) and electric conductivity were increased under PEG6000 treatment, which of the transgenic Medicago sativa were lower than those of the control. Under the osmotic stress, the chlorophyll and soluble sugar content of the transgenic plants were significant higher than those of control. By contrast a decreased content of proline was found in the the leaves of the transgenic plants. The results suggested that overexpressing BADH improved the osmotic resistance of Medicago sativa.