The content of essential oils and their composition in medicinal plants are affected by different factors, including genetic makeup and cultivation conditions such as climatic conditions, harvesting time, stresses and the use of fertilizers. In order to investigate the effect different nitrogen levels on yield components, seed and essential oil yield of fennel (Foeniculum vulgare Mill.), a field experiment was conducted based on randomized complete block design with three treatments (0, 50 and 100 kg.ha-1) and four replications at the Agricultural Research Station of Ferdowsi University of Mashhad, during growing season of 2009-2010. Nitrogen used as Urea in two stages (in the plating time and one month after then). Results showed that yield components, seed and essential oil yield of fennel were affected by nitrogen level. By inctreasing nitrogen application from zero to 100 kg.ha-1 seed yield was enhanced up to 23% and essential oil content and essential oil yield were declined up to 25 and 8%, respectively. Nitrogen is the cause of increasement of photosynthesis rate and duration which promote growth and yield. Also, nitrogen decreased essential oil content.

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