Abstract: The effects of Ni concentrations in the nutrient solution on the yield, quality and N assimilation of cucumber plants were evaluated in plants grown either with urea or nitrate as the sole N source. The cucumber plants (Cucumis sativus cv RS189 and Vikima) were treated with two N sources, urea and nitrate as NaNO₃ at 200 mg L^-1, and three concentration of Ni as NiSO₄.6H₂O (0, 0.5, and 1 mg L^-1). Treatments were arranged in a randomized block design with six replicates. The highest concentration of Ni in the leaves (1.2 mg kg^-1 DW) was observed in the urea-fed plants at 1 mg L^-1 Ni concentration. Addition of Ni up to 0.5 mg L^-1 had no effect on the fruit Ni concentration in the both urea and nitrate-fed plants. Ni supplement (0.5 mg L^-1) increased the yield significantly (10 and 15% in RS189 and Vikima, respectively), in urea-fed plants but decreased when 1 mg L^-1 Ni applied to the solutions. Nitrate-fed plants had higher percentage of total soluble solids compared to urea-fed plants. Nitrate concentration of the fruits in urea-fed plants in both cultivars was approximately 50% less than those nitrate-fed plants. The reduction of nitrate concentration in the fruits became more pronounced as the Ni concentration increased in the solution. The rate of photosynthesis (Pn) in urea-fed plants continuously increased with the increase of the Ni concentration in the solution. Both N concentration and NR (Nitrate Reductase) activity of young leaves were higher in urea-fed plants at 0.5 mg L^-1 Ni concentration. Ni supplements enhanced the growth and yield of urea-fed plants by the increase of Pn, N concentration and NR activity. It can be concluded that Ni supplements (0.5 mg L^-1) improves yield, quality and NR activity in urea-fed cucumber plants.