Background: Silybum marianum (L.) Gaertn. has been used for centuries as an herbal medicine for treatment of liver disease. Fruits of S. marianum contain silymarin, which is composed of the flavonolignans, silybin, isosilybin, silydianin and silychristin. Objective: The aim of this research was to investigate the effects of environmental conditions on silymarin content and morphological characteristic of S. marianum. Method: The dried fruits of S. marianum were collected from 13 area of Iran and cultivated at greenhouse conditions. Morphological characteristic were studied and silymarin content analyzed by HPLC. Results: The results showed that silybin is the main component of silymarin and was affected by environmental conditions. Conclusion: The amount of silymarin was very different in cultivated and endemic plants in comparison with together. The silymarin content had a significant positive correlation with seed yield/plant, stem diameter and number of capsules/plant.