Due to the limited water resources in agriculture, implementation of any strategies to conserve water and to increase land crop coverage is of great interest. One of these strategies is to use under irrigation method. One of the factors that play important rule in agriculture production is the availability of sufficient amount of nutrients especially nitrogen for plant use. To study the effects of different availabilities of irrigation water and nitrogen on sugar beet yield under T-Tape drip irrigation a field experiment was conducted at agricultural research center of Parsabbad, Moghan belonged to Ardebil province of Iran during the growing season of 2013-2014. The experiment was randomized complete block design with three replicates for each treatment. Irrigation treatments included: to apply 100% of crop irrigation water requirement (I 100 ), to apply 80% of the crop irrigation water requirement (IR 80 ) and to apply 60% of the crop irrigation water requirement (IR 60 ). These treatments were used as main treatments and four sub treatments of nitrogen fertilizer application were applied which they included: Optimal fertilization (N 100 ), 25 percent less than optimal fertilization (N 75 ), 50 percent less than optimal fertilization (N 50 ) and fertilizer application based on local farmers (N S ). The parameters of the mean weight of 10 roots from each plot, the mean diameter of 10 roots from each plot, the average length of 10 roots from each plot, number of roots, shoot weight, root yield, pure cutie, impure cutie, pure sugar yield and impure sugar yield were measured. The measured crop data were analyzed using SAS statistical software. The comparison of means was made using the LSD tests at the 5% level. The interaction effect of irrigation treatment with nitrogen fertilizer treatment on the mean weight of 10 roots from each plot, the average length of 10 roots from each plot, number of roots, root yield, pure cutie, impure cutie, pure sugar yield and impure sugar yield and water use efficiency were significant at the 5% level. The maximum and minimum root yields were belonged to IR 100 N 100 and IR 60 N 75 treatments with the yield values of 77083 and 55416 (kg/ha), respectively. The largest values of irrigation water use efficiency based on the root yield, pure sugar yield and impure sugar yield were belonged to the treatments of IR 60 N S , IR 100 N 100 and IR 60 N 100 with the values ??of 10.49, 0.586 and 1.068 (kg/m 3 ), respectively. The smallest value of irrigation water use efficiency based on the root yield, pure sugar yield and impure sugar yield was belonged to the treatment of IR 100 N S with the yield value of 6.09, 0.315 and 0.633 (kg/m 3 ), respectively. The results of optimization of applied irrigation water depth showed that using the total irrigation depth of 903 mm can achieve highest net income. Keywords: Defficit irrigation, Nitrogen, Sugar beet, Water use efficiency, Optimal water use