Community ecologists have focused on whether there exists an interactive effect between species diversity and genotypic diversity of dominant species on community ecological function in recent years. In the present study, we selected some common species in the Inner Mongolia Steppe as study objects, and examined how the genotypic diversity of the dominant species, the species diversity of common species and their interactions influenced the community biomass. (1) Genotypic diversity of Leymus chinensis, species diversity, and their interactions did not significantly influence the community aboveground, belowground, or total biomass (P > 0.05). (2) The genotypic diversity of L. chinensis, species diversity, and their interactions significantly influenced diversity effects (net diversity effect, complementarity effect, and selection effect) on community biomass (P < 0.05). The genotypic diversity of L. chinensis inhibited the net diversity effect, mainly complementarity effect. However, species diversity promoted the net diversity effect, in particular, selection effect on aboveground biomass. (3) The net diversity effect on ecological function of the community biomass was mainly due to complementarity effect. The above results provided not only an important message for exploring whether species diversity or genotypic diversity of dominant species plays a more important role in influencing the diversity effect on community biomass, but also proper guides for protection and rational utilization of seed resources, and moreover, for the ecosystem recovery and re-establishment in the Inner Mongolia Steppe.