the effect of manganese and salicylic acid on gene expression of Menthone Reductase (MR) and menthol content in Mentha piperita were investigated. The research was conducted by a factorial experiment design in a randomized complete block with three replications. The experimental factors include the treatments by manganese (500 µM) and salicylic acid (1 mM) and time (1, 3 and 5 days) after spraying. The results of ANOVA indicated that the manganese and salicylic acid treatments had significant effect on gene expression of Menthone Reductase and menthol over all three times after spraying. The lowest gene expression of MR and menthol casused by manganese at 5 days after spraying was significantly different from control. The maximum gene expression of MR and menthol content was achieved by salicylic acid at 5 days after spraying. The analysis of the interaction of manganese and salicylic acid showed that salicylic acid reduced the manganese toxicity to some extent in the expression levels of MR and the menthol content. The results of this experiment showed that a direct correlation exists between the MR expression and the amount of menthol in peppermint, so that over time by increasing the MR expression, the amount of menthol was also increased by salicylic acid while the expression of MR and the amount of menthol was decreased by manganese. However, it was observed that salicylic acid acts as a powerful hormone and stabilizes the effect of manganese on MR gene expression and menthol content in peppermint plant.