Background: Environmental factors have obvious effect on the growth, yield and quality of effective materials of the medicinal plants, then it is necessary to study the factors effectiveness on the medicinal plants and specially the nutrition and fertilizer are important considerably. Fennel is one of the most important medicinal plants in Iran which is applied in pharmacy and cosmetics care industries very much.

Objective: Study of effectiveness nitrogen levels on qualitative and seed yield in fennel .

Methods: This investigation was carried out as splite plot design with three replications and N fertilizer and fennel population factors in spring 2008. Main factor included fertilization levels (0, 40, 80, 120 and 160 kg/ha N) and sub factor were fennel populations (Esfahan, Tehran, Yazd and 11486).

Results: There was significant difference among populations for essential oil% content in seed, essential oil% in aerial parts, protein%, ash% and seed yield. The effect of different N levels was significant on the essential oil% content in seed, essential oil% in aerial parts, protein%, fiber% and seed yield.