Group sizes of Lasiopodomys brandtii overwintering colonies could affect colonies’ predation risks. Using the number of barrows of Brandt’s vole colony as an indicator of family group size, we studied how group sizes of Brandt’s vole’s affect the predation preference of Buteo hemilasius’s in Xilinguole typical steppe, Inner Mongolia, in the fall of 2004. The result shows that Buteo hemilasius significantly prefers to high number of group sizes of vole’s colonies. This finding indicates that the colonies with larger size result into a higher predator risk. It explains why Brandt’s vole dispersal happened during fall, Brandt’s vole colonies choose to reduce their group sizes to avoid predation risk. Such result also supports the optimum group size theory of small mammals.