In order to study the effects of nitrogen fertilizing on the biomass and its allocation of different grassland plants and thus on the carbon exchange in temperate grassland ecosystem, potted control experiments for four grassland species, i.e. Medicago sativa, Sorghum bicolor, Leymus chinensis, and Caragana microphylia, were conducted under three nitrogen levels, which were the control (CK) with no nitrogen addition, the medium (N1) with 30gN/m² addition, and the high (N2) with 60gN/m² addition, at Taipusiqi Grassland-Cropland Ecosystems Experiment Station of Inner Mongolia. Medicago sativa is a perennial legumes grassland species, Caragana microphylia is a legumes shrub, Leymus chinensis is a perennial C3 plant, and Sorghum bicolor is an annual C4 species. The results show that the above-ground biomass of all the four species increased significantly after nitrogen addition, with the largest above-ground biomass occurred under N1 level for Medicago sativa (with 24.8% higher than CK), but under N2 level for the other three species (with 45.6, 39.3, and 72.2% higher than CK for Sorghum bicolor, Leymus chinensis, and Caragana microphylia, respectively). The stem biomass of Sorghum bicolor, Leymus chinensis, and Caragana microphylia was also increased significantly under increasing nitrogen level. The leaf biomass reached the largest under N1 level for Medicago sativa and Caragana microphylia (98.47g/m² and 143.87g/m²), but under N2 level for Sorghum bicolor and Leymus chinensis (69.20g/m² and 95.07g/m²). In contrast, the underground biomass of all the four species was the largest under N1 level. The biomass of coarse roots (with diameter >2mm) of Medicago sativa, Sorghum bicolor, and Caragana microphylia didn't increase significantly, while the rhizome biomass of Leymus chinensis increased significantly. However, the biomass of fine roots (with diameter ≤2mm) of all the four species decreased greatly with higher nitrogen level. Overall, the aboveground biomass and fine root biomass of the four species were sensitive to nitrogen addition. The root/shoot ratio of all the four species declined after nitrogen addition, with the lowest ratio under N1 level for Medicago sativa (1.62), and under N2 level for Sorghum bicolor, Leymus chinensis, and Caragana microphylia (0.57, 1.02, and 0.41 respectively). The stem/leaf ratio of Medicago sativa and Leymus chinensis decreased with increasing nitrogen addition, while that of Sorghum bicolor and Caragana microphylia increased. In summary, the four species, even the two legumes had different response to nitrogen addition, i.e., higher nitrogen addition could increase the biomass of gramineous species significantly and change their biomass allocation pattern obviously, while the aboveground and underground biomass of Medicago sativa didn't increased significantly, and the biomass allocation pattern didn't change. The rhizome and leaf biomass of Leymus chinensis increased greatly with nitrogen addition, while Sorghum bicolor and Caragana microphylia allocated more biomass to stem.