Background: The use of ultraviolet C radiation is a low-cost technique that is easy to apply to plants, and is widely used for disinfection in many industries and food processing.  The milk thistle seeds have medicinal properties and antioxidant and anti-inflammatory effects.  The aim of this work is to study the effect of ultraviolet C on silymarin content, germination growth and internal structure of milk thistle medical seeds.  Methods: The seeds of milk thistle were received from the Egyptian Ministry of Agriculture, and used as research samples.  The ultraviolet irradiation system consists of one fluorescent lamp (type- C) which is covered totally with aluminum foil to illuminate the sample from all sides.  The structure of the used samples was studied by x-ray and IR analysis.  Results: The concentration and yield values of silymarin in various extracts for milk-thistle seeds increased after being exposed to UVC at different times at dissimilar distances.  The weight and germination height values of irradiated milk thistle seeds by UVC are much less than untreated seeds.  From x-ray and IR analysis, it is obvious that the arrangements and position of milk-thistle molecules changed after being exposed to UVC for different times and distances.