Along with the increasing scarcity of global water resource and gradual seriousness of drought, there is an urgent need of developing water-saving technologies, such as intermittent irrigation or even aerobic cultivation of rice. Moreover, the rhizosphere of rice roots is actually in partial oxidized status due to the released oxygen by rice roots. In well-drained soils, ammonium (NH₄⁺) converts rapidly to nitrite and then to nitrate (NO₃^-), so NO₃^-nutrition is becoming more and more important for N nutrition of rice plants. Solution culture experiments were carried out to study the effects of NO₃^- on the yield of two rice cultivars with different nitrogen use efficiency (NUE). For more mechanisms of responses of rice to NO₃^-, yields components, rice growth, N uptake and assimilation were also studied at different growth stages. Nitrification inhibitor, dicyandiamide (DCD) was applied to prevent nitrification and denitrification in nutrient solution. The results obtained were as follows. Compared with those of 100/0 NH₄⁺-N/ NO₃^--N, a ratio of 75/25 NH₄⁺-N / NO₃^-N increased the spikelets per panicle of Nanguang cultivar (high NUE) and increased its grain yield by 21%, while the yield of ELIO cultivar (low NUE) has no significant differences in the two nutrient solutions. NO₃^-addition increased total N accumulation and dry matter production in Nanguang by 36% and 30%, respectively, averagely for the four growth stages, while the increased effect of NO₃^- was not found in ELIO cultivar. In the mixture of NO₃^-and NH₄⁺- nutrient solution, the nitrate reductase activity in the leaves and glutamine synthetase activity in the roots of Nanguang were increased by 100% and 95%, respectively, compared to the 100% NH₄⁺- treatment. All these results showed that the increased spikelets per panicle and improved nitrogen uptake and assimilation by NO₃^- might contribute to the increased grain yields of Nanguang. Rice cultivar with high NUE has stronger response to NO₃^-than the rice cultivar with low NUE, suggesting that there might be a relationship between NO₃^-nutrition and NUE.