Field and sand culture experiments were conducted to study varietal difference of rice plants in response to N levels and its physiological mechanisms. The results obtained show that at low N (mediately deficiency) and high N (sufficient) levels the biomass of the top and grain yield of the varieties tested at main growth stages decreased in the order: Shanyou 64 > Shsnyou 6 > Zheli 1 > Xiushui 48. The greatest difference was found between the hybrids and the conventional varieties, especially at a low N level. The results also show that the rice variety which could obtain higher yield at a low N rate had a greater potential of absorbing and utilizing N from the soil with following characteristics: 1) a well developed and vital root system with a greater root volume and distribution density, and a higher affinity to NH₄⁺, i.e. with a smaller Ko value; 2) greater activities cf the key enzymes involved in the assimilation of NO₃^- and NH₃ and photosynthesis in the leaves. All these physiological characteristics may be used as the indices for identifying and selecting N-efficient variety or genotype.