Abstract: In the grassland region of Inner Mongolia, herdsmen always mow tall grass in autumn in preparation for feeding livestock in winter. The grassland has become increasingly degenerate because of annual mowing so we studied the effects of N addition to mown grassland on standing crops, species composition, community structure, and nitrogen content of the main populations in Stipa grandis grassland of a typical steppe. 1) For plots mowed once a year, the community standing crops after adding N were higher than those of control plots, and the plots with more N (N3, 64.4 kg/ha, N4, 78.2 kg/ha) also had significantly higher productivity in the first two years after adding N. For plots mowed twice a year, only the plots with highest N addition (N4) had significantly higher standing crops than the controls. N addition to non-mowed plots had some effect although this was not significant. 2) Community species diversity of the two mown systems increasingly fluctuated with the added N except for the non-mown system, and generally, the evenness decreased with increased N addition. 3) For the three main populations, two aspects (crops and nitrogen content) affected by adding N were considered. First, after adding N, S. grandis and Leymus chinensis had equal and super compensation on standing crops in control plots and plots mowed once a year, but in plots mowed twice a year there was no compensation, while with Cleistogenes squarrosa standing crops had equal compensation to control plots, but super compensation in plots mowed after adding N. Second, nitrogen contents in S. grandis and L. chinensis plants were increased with the added N whether in plots mowed once a year or in non-mowed plots. But for C. squarrosa, this phenomenon could only be found in plots mowed once a year, while in non-mowed plots, nitrogen content was highest in the N2(50.6 kg/ha)plots and maintained this level with higher application rates. 4) N addition not only ensured productivity of grassland mowed once a year, but also improved the grassland quality to some degree. The N4 treatment had the best effect on grassland.