In order to investigate the effect of Piriformospora indica endophyte fungi on quantity and quality of peppermint essential oil and physiological parameters under salt stress, An Research Field experiment of Agricultural Sciences and Natural Resources University of Surrey in 2015 was conducted. Experiment arranged in completely randomized design with three replicates. The experiment was done in factorial arrangement based completely randomized design with three replications. Treatments were application Piriformospora indica (including control and inoculation with P. indica) and four salinity levels of Caspian Sea water (0, 3, 6, 9 dS/m). ). Results showed that increasing of salt stress significantly decreased root colonization, leaf essential oil content, chlorophyll a, chlorophyll b, carotenoids, plant dry matter yield and relative water content, and enhance the attributes of The Secondary Metabolites such as Menthol, Menthone and Methyl acetate, soluble sugars and percentage of electrolyte leakage when salt stress was increased. Fungi inoculation, however, markedly improved Total dry weight and photosynthetic pigments. Biological treatment also reduced the negative effects of salinity on leaf essential oil content, cell membrane stability and relative water content of leaf. Overall, it seems that applied P. indica Caused increased the essential oil of peppermint By stimulating plants to increase synthesis of secondary metabolites, And extending root system And increased water and nutrient absorption By fungus hyphae. In conclusion, our results reprsented the ameliorate effect of P. indica in saline conditions, and could increase resistance to stress, greatly improve the growth characteristics of peppermint.