Peppermint plants were initiated from rhizome cuttings (10 cm long) supplied by Jahad Daneshgahi in March 2009. According to the method described by Farahani et al., (2009), seventy five peppermint samples were notified above were transferred into pots (25 cm head diameter × 20 cm height) filled with sandy loam soil (soil average amount of 5580 gr in each pot). This experiment was carried out using a randomized compllete block design with 3 replications. The factors studied included 5 levels of water deficit stress that were D1 (100% field capacity), D2 (85% field capacity), D3 (70% field capacity), D4 (60% field capacity) and D5 (45% field capacity), respectively. In order to determine the soil moisture rate of 100% field capacity, 24 h after irrigation we selected 5 field soil samples by sampling drill, then samples were weighed by electrical scale and placed under 105 ℃ in electrical oven for 48 h. Field soil samples were selected for determination of soil moisture rate daily, than determined 85% field capacity, 70% field capacity, 60% field capacity and 45% field capacity, respectively.

Results indicated that drought stress motivated a significant reduction in all of the growth parameters and essential oil yield and percent. The highest values of growth parameters and essential oil percent and yield were observed under 100% field capacity (control). Also, the highest values of Mentone and menthofuran were obtained under 100% field capacity (control) and the highest values of menthol were obtained under 70% field capacity by using gas chromatography-mass spectroscopy (GC-MS).