Nickel (Ni) is a structural component of urease enzyme and it is required for nitrogen metabolism in higher plants. Deficiency of Ni results in reduction of urease activity and probably other enzymes responsible for nitrate reduction. Little is known regarding the growth and physiological response of onion plant to Ni nutrition. In this research study we investigated the effects of Ni on nitrogen metabolism and growth of two onion cultivars ( Allium cepa L., cvs. Dorrcheh and Cebolla Valenciana ) supplied with ammonium nitrate or urea as nitrogen source. Three levels of nickel (0, 25, and 50 µM) were used in the form of NiCl 2 or Ni-histidine [ Ni(His) 2 ] complex. Results obtained from the present study showed that in plants supplied with urea as sole nitrogen source, aaddition of Ni in the forms of NiCl 2 or Ni(His) 2 complex resulted in increase of urease activity and decrease of bulb urea concentration. Addition of Ni into the nutrient solution, regardless of its applied source, stimulated hydrolysis of urea and production of NH 4 + and resulted in higher concentration of the total amino acids and nitrogen in onion bulbs. The efficiency of Ni(His) 2 complex in improving Ni uptake and increasing activity of urease and glutamine synthetase (GS), two enzymes involved in urea metabolism, was greater than NiCl 2 . Results also indicated that Ni nutrition resulted in enhancement of onion bulb yield probably due to the positive effect of Ni on improvement of nitrogen metabolism in plans grown at the nutrient solutions containing urea. For onion plants supplied with ammonium nitrate as nitrogen source, addition of Ni into the nutrient solution positively affected nitrate metabolism and resulted in lower accumulation of nitrate in the bulbs. Addition of Ni into the nutrient solution, regardless of its source, improved bulb yield of onion cultivars via stimulating reduction of nitrate and increasing concentrations of NH4+ and total nitrogen. In nutrient solution containing ammonium nitrate, the efficiency of Ni(His) 2 complex in improving Ni uptake, increasing activity of nitrate reductase (NR) and glutamine synthetase (GS), increasing total nitrogen concentration and reduction of nitrate accumulation was greater than NiCl 2 . According to the results obtained from the present study, Ni nutrition particularly in the form of Ni(His) 2 complex as a modern Ni fertilizer source in the nutrient solution decreased urea and nitrate concentration in onion bulb and thereby, increased yield and health quality of onion. Key words : Nickel, Histidine, Onion, Nitrogen metabolism, Urease, Nitrate reductase (NR), Glutamine synthetase (GS).