Based on field survey and laboratory experiments, we compared soil organic carbon density and aboveground biomass of three types of grassland (meadow grassland, typical grassland and desert grassland) in Inner Mongolia, northern China, in order to provide reference for grassland protection and improvement of soil organic carbon sequestration capacity. The results are as follows. 1) The variation regularity of soil organic carbon density differs in each of the three types of grassland: it is 100 -180 g for the meadow grassland, 120 -140 g for the typical grassland and 60 -80 g for the desert grassland, while it is significantly different when the biomass is beyond the threshold value. 2) For different communities at the same biomass levels (80 - 100 g and 120 - 140 g), there exist some differences in soil organic carbon density, but without any obvious regularity. In brief, the improvement of biomass and prevention of grassland degradation would have positive effects on the increase of soil organic carbon density and improvement of soil organic carbon sequestration ability of grasslands.