Effect of penconazole (PEN) on the expression level of two genes in the biosynthesis pathway of monoterpenes, isopiperitenone reductase (iPR) and pulegone reductase (PR), and essential oil (EO) compounds were studied at flowering stage of Mentha pulegium L. under drought stress. Plants were grown with different levels of field capacity (100 and 50 %) with or without PEN (15 mg l⁻¹). Results showed that drought stress decreased the growth and productivity parameters. PEN treatment to drought-stressed plants decreased the negative effects of drought stress on these parameters. The EO yield increased by about 1.6 times under drought stress, and the highest amount of EO was obtained in drought-stressed with PEN. Drought stress increased pulegone and decreased menthone percentage, and the highest pulegone percentage (78.2 % of total constituents) was obtained in drought-stressed with PEN treatment. Semi-quantitative RT-PCR showed drought stress increased the expression level of iPR and PR genes. PEN treatment promoted the impact of drought stress on iPR gene expression and repressed PR gene expression. Our results suggest that PEN may be a useful tool for the regulation of monoterpene metabolism in M. pulegium under stress condition.