Abstract: Increasing atmospheric CO₂ concentrations is generally expected to enhance photosynthesis and growth of agricultural C₃ vegetable crops and, as a result, substantially increase yields. However, little is known about the combined effect of elevated CO₂ and N species on plant growth and development. A growth chamber experiment was conducted to determine the influence of NH⁺₄/NO^-₃ ratio and elevated CO₂ concentrations on the growth and development of tomato seedlings. Tomato (Lycopersicon esculentum Mill var. Hezuo 906) was grown for 45 days in pots with nutrient solutions varying in NH⁺₄:NO^-₃(%) ratio (100∶0, 75∶25, 50∶50, 25∶75, 0∶100) in growth chambers with ambient(360 μL/L) or elevated CO₂ concentrations(720 μL/L). Elevated CO₂ increased photosynthetic rate and water use efficiency of tomato seedlings in all NH⁺₄/NO^-₃ ratios in nutrient solution except ammonium-fed plants. Photosynthetic rate and water use efficiency of tomato seedlings were increased in the elevated CO₂ treatment with increasing proportion of NO^-₃ in nutrient solutions and and photosynthetic rate were up to 55% higher at elevated than at ambient CO₂ concentration. At both CO₂ levels, the photosynthetic rate and water use efficiency of the plants increased as increasing proportion of NO^-₃ in the nutrient solutions. The results indicate that tomato seedlings may benefit more from CO₂ enrichment when increasing proportion of NO^-₃ in nutrient solutions is supplied, and elevated CO₂ does not inhibit the adverse effects on tomato seedlings when 100% NH⁺₄-N in nutrient solution is supplied. In all, elevated CO₂ concentration increased water saving on agricultural production and water productivity and at two CO₂ levels, NO^-₃-N was the most suitable N source to tomato seedling growth, when other NH⁺₄/NO^-₃ ratio was supplied, the growth of tomato seedling was inhibited to some extent. When the N source supplied as solely NH⁺₄-N, tomato seedling was hard to grow normally.