Characteristics of nitrogen absorption and response to nitrogen dosage of rice seedlings of 12 rice cultivars selected out through previous field experiments were studied hydroponically. Results showed that the differences in nitrogen use efficiency between the 12 cultivars existed not only in field conditions but also in hydroponical conditions, and even cultivars of the same genotype performed differently at the seedling stage in hydroponical conditions. N-efficient cultivars, GD4, YJ38 and QY421, still performed well at the seedling stage, but N-efficient cultivars NG, YJ7, 4007 and HD did poorly; Among the three N-inefficient cultivars, Elio, FNXHM and HJN, Elio was outstanding in performance. Cultivars of GD, QY421, Elio and YJ38 were identified as N-efficient ones while NG, 4007 and WYJ7 as N-inefficient groups. The typical N-efficient cultivars of GD, QY421, Elio and YJ38 showed characters of higher biomass, higher nitrogen absorption and higher response to nitrogen dosage, and among them, GD, QY421 and Elio could make a rapid increase in Vmax from 0.5 mmol/L NH₄⁺-N to 1.0 mmol/L NH₄⁺-N and the higher biomass of YJ38 was crucial to its nitrogen uptake; The typical N-inefficient cultivars of NG, 4007, WYJ7 and YJ7 were characterized as lower biomass, lower nitrogen absorption and lower response to nitrogen dosage, and among them, the Vmax of NG, 4007 and WYJ7 decreased drastically and the Km increased smartly from 0.5 mmol/L NH₄⁺-N to 1.0 mmol/L NH₄⁺-N. Therefore, biomass, especially roots weight, affinity to NH₄⁺ and Vmax are the three important factors for nitrogen efficiency of rice seedlings.