Vigilance behavior, affected by many factors, is animals′ response to potential dangers. To study the vigilance behavior of Brandt′s vole (Lasiopodomys brandtii), we examed the effects of group size and foraging distance on the vigilance behaviour frequency of Brandt′s vole during the food storing period in typical steppes in Xilingole League, Inner Mongolia. We manipulated the group size of the voles in three levels (11, 6, and 3 individuals) and foraging distance in four levels (5 m, 10 m, 20 m, 30 m) in this study. The vigilance behaviour frequency was taken counted in the same time (Fig. 1 and Appendix). Two-way ANOVA shows that the frequency of vigilance behaviour significantly related to both group size and foraging distance. Moreover, the interaction between group size and foraging distance was extremely significant (Table 2). Vigilance behaviour frequency was higher with smaller group size and longer foraging distance. The result suggests that the group size effect of Brandt′s vole well supports the many-eyes hypothesis. And the result also indicates that foraging efficiency of Brandt′s vole rises with the increase of the group size. Combined with the study that with higher group size comes higher predation risk, the trade-off between predation risk and foraging efficiency makes us ensure that there is an optimum group size of Brandt′s vole.