Nickel (Ni) nutrition plays an important role in increasing the efficiency of nitrogen use and improves growth, yield and quality of crops. The beneficial effect of nickel application on urease activity and increased the hydrolysis efficiency of urea as a source of nitrogen in plants, has been studied extensively in previous studies, but there is limited information regarding the effect of Ni on activity of other enzymes involved in the plan nitrogen cycle. In this study, at the first step, urea–Ni complex was synthesized and then the efficiency of foliar application of this complex in terms of the impact on nitrogen metabolism, yield and nutritional quality of lettuce was compared with NiCl 2 . For this purpose, two cultivars of lettuce ( Lactuca sativa L. Conco Istador and Grizzly) were grown in nutrient solutions containing urea and ammonium nitrate. Nickel was used as foliar spray application at a concentration of 25 mM in the forms of NiCl 2 and urea-Ni complex twice a week. Two weeks after exposure to Ni treatments, plants were harvested and the fresh and dry weight of roots and shoots, concentrations of iron (Fe), zinc (Zn), urea, nitrate (NO 3 - ), ammonium (NH 4 + ), Ni and chlorophyll a and b and activity of urease and glothamin synthetase (GS) in the shoots were measured. Based on the results, Ni nutrition, irrespective of the nitrogen source, increased the fresh and dry weight of roots and shoots of both lettuce cultivars; although the effect of Ni on plants fed with urea was significantly higher than those fed with ammonium nitrate. Also, foliar application of Ni increased Ni concentration in shoots of lettuce cultivars in comparison with the Ni-free control. Effect of urea-Ni on increasing the concentration of Ni in plants fed with ammonium nitrate was significantly higher than those fed with NiCl 2 . Nickel nutrition increased urease activity and reduced urea concentration in both lettuce cultivars. Foliar application of Ni also reduced tissue concentration of NH 4 + by enhancing activity of glothamin synthetase (GS). C oncentration of nitrate in plants treated with Ni was lower than those untreated with Ni probably due to the stimulating effect of Ni on activity of nitrate-reductase enzymes. Foliar application of Ni also increased concentrations of chlorophyll a and b in lettuce leaves. The efficiency of urea-Ni complex in supplying Ni and improvement of nitrogen metabolism was greater in plants at the nutrient solution containing urea than those at the nutrient solutions containing ammonium nitrate. In this study, foliar application of only urea, at a concentration equivalent to the amount used with Ni-urea complex had no significant effect on the yield of two lettuce cultivars. Therefore, the greater stimulating effect of Ni-urea complex on the growth and yield of lettuce cultivars in comparison with NiCl 2 seems to be due to the role of Ni rather than urea. Keywords : Nickel, Complex, Urease, Glothamin synthetase, Nitrogen metabolism, Foliar ray, Lettuce.