Abstarct: Salvia leriifolia is one of the valuable medicinal plants of labiataeand native to Iran. To identify the effect of melanin on the morphological characteristics, antioxidant properties and some biochemical traits of in vitro Norozak plantlets under polyethylene glycol osmotic stress were conducted. Analysis of the results showed that drought stress led to an increase in fresh and dry weight of roots, malondialdehyde, hydrogen peroxide, proline, total phenol, and total flavonoid, activity of PAL and TAL enzymes, antioxidant capacity and antioxidant enzymes. Oxidative and also decrease in fresh and dry weight of shoots, shoot and root length, root diameter, leaf length and width, photosystem II quantum function and protein content, which was more significant in 5% polyethylene glycol concentration. Also, the use of melatonin in this research resulted in improvement in the parameters of stem diameter, crown diameter, root tip diameter, root fresh and dry weight, leaf and petiole length, Fv/Fm, protein, proline and CAT antioxidant enzyme activity. , but in the parameters of stem and root length, root diameter, shoot dry weight, number of leaves, malondialdehyde and TAL enzyme, a decreasing trend was observed with the use of melatonin. In the enzyme SOD, PAL and H2O2, only the concentration of 100 mg/liter of melatonin showed an increase compared to the control, while the concentration of 10 mg/liter decreased these parameters. In general, the results indicated that melatonin reduces the drought stress in the Salvia leriifolia plant by activating the antioxidant defense system.