Water stress is one of the most important factors that restricts wordwide productivity of crop plants. Two experiments at pot and field conditions were carried out in the College of Agriculture, Isfahan University of Technology. In the first experiments, a completely randomized design (CRD) was conducted with four levels of water stress inducing soil water potential of -0.33, -4, -8 and -12 bar. Each treatment was replicated four times, and each replication was provided with four pots using two plants in each pot. In the second experiment, five treatments inducing soil water potential of –0.33, -4, -8, -12 and –15 bar were applied. treatment were applied using a randomized complete block design with four replications. Results indicated that water stress reduced plant length, number of leaf per plant, shoot fresh weight, shoot dry weight, number of capitule per plant, axillary capitule diameter, seed yield in terminal and axillary capitule and one thousand seed weight in both experiments. Drought caused a significant reduction in root fresh and dry weight, root length and diameter in pot experiment. The results indicated that main capitulue diameter was the only morphological trait, which was not significantly influenced by water stress levels in pot and field experiments. The highest content of Silibin a, b and total amount were measured with -4 and -12 bar in pot and field experiments respectively. The highest content of Silymarin was accumulated under water potential of -4 bar in both experiments. The results demonstrated no significant different among the treatment in terms of antioxidant activity. However, The highest antioxidant activities was observed in -12 bar treatment in the field conditions. The highest oil percentage was obtained with -4 bar in both experiments. However, increase in drought level and time caused decrease in leaf chlorophyll content and leaf chlorophyll florescence (Fv/Fm). The highest leaf chlorophyll content and leaf chlorophyll florescence (Fv/Fm) was obtained in control treatment and 2 weeks after water stress and control in 6 weeks after water stress in pot expriment, respectively. The highest leaf chlorophyll content was observed in -8 bar, 2 weeks after waters stress treatment which exhibited no significant difference with control treatment at 2, 4 and 6 weeks after water stress treatment in the field experiment and the highest ratio of leaf chlorophyll florescence (Fv/Fm) was observed 2 weeks after water stress. In both experiment, the lowest leaf chlorophyll content and leaf chlorophyll florescence (Fv/Fm) was obtained in -12 bar, 6 and 8 weeks after stress treatment. The results of this experiment showed that secondry metabolites were increased under water deficit. Also according to the results of this study, it can be expressed that Milk Thistle plants under field conditions were more resistance to water stress than pot conditions. Keywords: Milk Thistle, Silymarin, Silibin, Antioxidant Activities and Water Stress .