Drought stress is one of the most important non-polluting environmental stresses affecting crop production. In order to identify tolerant fennel populations to water deficit stress and determine the most effective drought tolerance indices, an experiment was conducted in randomized complete block design with three separate replications under limited and normal condition of irrigation with 19 fennel populations at the Agricultural Research Station, University of Tabriz in 2016. Water deficit stress applied during the beginning of flowering by disruption irrigation. Routine irrigation performed for normal condition. Combined analysis of variance showed that there is significant difference (p ≤ 0.01) between two irrigation condition and also populations for most traits. Population × condition interaction of most traits was significant (p ≤ 0.01). The results showed that the number of umbels per plant, number of grain per umbel, grain weight, grain yield and essential oil yield in all evaluated populations are reduced under water deficit stress conditions. While the essential oil of the whole population increased in the water deficit stress conditions. Evaluation of population from the drought tolerance was performed by six different index SSI, TOL, MP, STI, GMP and HARM. According to positive correlation indices MP, GMP, HARM and STI with grain yield and essential oil yeild, under normal and water deficit stress condition of irrigatin this four indicators were identified as the most effective index for determining drought tolerant populations. According to the four major indices, Bonab, Birjand, Germany (11486) and Hamedan populations were identified as water deficit stress tolerant and Qazyantp and Khorshidabad populations were identified as sensitive to water deficit stress. The comparison results showed that Bonab population was the best a population from the important characteristics such as grain yield, essential oil yield, umbels per plant, grain weight and biomass under normal irrigation conditions and from the umbel per plant under stress conditions.

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