Abstract: The ripe fruit of milk thistle (Silybum marianum L. Gaerth) contain flavonoids that are used to prepare anti - hepatotoxic drugs. This plant is important to pharmaceutical industries. The main aim of this investigation was to study the influence of nitrogen fertilizer on growth, seed yield and active substances (silymarin and silybin) content of milk thistle. The results showed that nitrogen fertilizer had significant effect on growth (plant height, number of capitula per plant, capitula diameter), seed yield, silymarin and silybin content. The highest seed yield (2.35 kg/plot) was obtained from the plots, receiving 200kg / ha N as top dressing after seed germination. But the highest silymarin (9.25%) and silybin (33. 58%) content was accumulated in the seeds of control treatment plants.