The aim of this study was to determine the effects irrigation interval and sodium selenate foliar application on phytochemical traits of milk thistle. Irrigation regimes consisted of 30, 60 and 90 mm evaporation from class A Pan and the second factor is sprayed with sodium selenate 0, 25 and 30 mg/L. The chemical composition of the essential oil was examined by GC and GC-MS and they were significantly affected by Sodium Selenate Foliar and Irrigation Intervals. The results proved analysis of variance indicated that the effects of drought and the interaction between sodium selenate spraying and irrigation had significant effect on leaf proline, oil content, silymarin and silybin percent. The total oil content, silybin and silymarin decreased by 2% due to increasing the water scarcity but the amount of the proline increased under the drought stress. The highest amount of the silybin (19.7%) and silymarin (3.617%) and oil content (19.53%) were measured in treatment 30 mm evaporation from pan and spray 25 mg sodium selenate whereas the highest amount of the proline (13.79%) in treatment of 60 mm evaporation from pan and 30 mg spray sodium selenate. It has proved that the drought stress increased the quality of milk thistle seed oil.