A field experiment was conducted at Tameia Agric.Res. station, Fayoum Governorate, during 2010/2011 season and repeated in 2011/2012 one, aiming at studying the effect of irrigation regimes as different available soil moisture depletion (ASMD) and N - fertilization levels as (100,120 and140 Kg.N fed^-1 soil – injected ammonia gas) on onion bulb yield, yield components and some crop - water relations. To achieve the previous targets, N fertilization levels i.e. 100, 120 and  140 kg N/fed were combined with three irrigation regimes i.e. irrigation at 35, 55 and 75% ASMD in a strip-plot design with four replicates. The main obtained results could be summarized as follows:- 1. Onion bulbs yield and yield components were significantly affected by the adopted both N fertilization levels and irrigation regimes and their interaction as well in both seasons. 2. The highest averages of bulb weight, bulb diameter and bulbs yields (18.750 and 17.910 t fed^-1) were detected from applying 140 kg N fed^-1 and irrigation at 35% ASMD interaction, whereas the lowest figures resulted from the interaction of 140 kg N fed^-1 level and irrigation at 35% ASMD, in 2010/2011and 2011/2012 seasons. 3. Seasonal evapotranspiration (ET_C)., as a function of N -fertilization levels and irrigation regimes were 35.03 and 33.85 cm in 2010/2011 and 2011/2012 seasons, respectively. The highest ET_C values (40.32 and 39.11 cm) resulted from applying 140 kg N fed^-1 and irrigation at 35% ASMD in the two successive seasons. Applying 100 kg N fed^-1 and irrigation at 75% ASMD gave the lowest ET_C values which comprised 29.93 and 29.06 cm in 2010/2011 and 2011/2012 seasons, respectively. 4. The crop coefficient (K_C) values were 0.56, 0.69, 0.79, 0.94 and 0.74 for December, January, February, March and April, respectively, ( two seasons average of the highest yielding interaction e.g. supplying N at 140 kg fed^-1 rate and irrigating at 35%ASMD). 5. The highest water use efficiency values in 2010/2011 and 2011/2012  seasons (10.054 and 10.903  kg dry bulbs m^-3 water consumed, respectively) were detected from applying 140 kg N fed^-1 and irrigation at 35% ASMD.