Limited information is available on biological effects of various levels of nickel (Ni) (deficiency to toxicity levels) on growth and yield of certain crops, particularly vegetables. In this sand-culture study, we investigated the effects of four levels of Ni (0, 50, 100, and 200 μM) on growth, yield, and fruit-quality attributes of two cucumber cultivars (Cucumis sativus L. cvs. Super Dominus and Negin) supplied with urea or NH4NO3 as nitrogen source. Addition of 50 μM Ni to the nutrient solution resulted in a significant increase of shoot and root dry-matter yield of cv. Negin although this increase was greater in the urea-fed plants than those fed with NH4NO3. In both cultivars, addition of 50 μM Ni increased urease activity and thereby decreased the urea concentration in the urea treatment. Addition of 100 and 200 μM Ni caused a significant decrease in root and shoot growth of cucumber although this decrease was insignificant for cv. Super Dominus in the 100 μM treatment. The highest fruit yield, total soluble solids (TSS), and fruit firmness were achieved at the 50 μM Ni treatment. Regardless of nitrogen source, Ni addition proportional to the concentration used increased leaf Ni concentration and fruit acid ascorbic concentration. The concentration of Ni required for optimum growth and yield of cucumber varied with cultivars. The level of 50 μM was sufficient for optimum growth of cv. Negin in nutrient-solution culture while lower concentration of Ni was required for cv. Super Dominus. While the beneficial effects of sufficient levels of Ni on growth and yield of urea-fed plants was greater than with NH4NO3-fed plants, the toxic effects of Ni in these plants were also greater.