Restriction in gas exchanges is one of the reasons in reduction of the plant growth in water scarcity and drought stress conditions. In this study, the effect of drought stress on gas exchange characteristics of 36-two year old Persian walnut (Juglans regia L.) seedlings were studied in complete randomized design with nine replicates and four treatments. In terms of drought, irrigation treatments 100 (control), 75, 50, and 25 percent of the potential water were applied according to water requirement of walnut in field capacity. Photosynthesis, transpiration, stomatal conductance, and CO₂ concentration were measured before and after the stress using IRGA gas exchange instrument. Based on the analysis of variance, photosynthesis, stomatal conductance and mesophilic conductance had significant difference among irrigation treatments. The statistical comparison with Duncan's multiple range test showed that the rate of photosynthesis and stomatal conductance at 25 and 50 percent of irrigation treatments had significant differences with the control. Significant difference was also observed for mesophilic conductance between 50 percent and control treatment. Meanwhile, there was no significant difference for all measured parameters between 75 percent and control treatment. Therefore, saving water for irrigation of the plants can be reduced to 75 percent of field capacity.