Although temporal partitioning has long been viewed as a valid mechanism in reducing competition, direct field evidence is rare, and the temporal niche is often considered less important than other factors in promoting coexistence. Furthermore, the relative importance of biotic and abiotic factors in shaping diel activity patterns of animals is poorly understood. We conducted controlled experiments in a little-studied area (Hulunbuir meadow steppe, China) to explore the effects of presence of a larger species, Spermophilus dauricus, and ambient temperature in shaping diel activity patterns of Microtus gregalis. Our results suggest that although interference competition existed between S. dauricus and M. gregalis, the presence of S. dauricus did not significantly affect the diel activity patterns of M. gregalis (P > 0.05). Timing of activity of M. gregalis, however, was always significantly correlated with ambient temperature (P < 0.001). As winter approached, ambient temperature appeared to be a major factor in shaping activity patterns of M. gregalis. To our knowledge, this is the 1st experimental study conducted in an East Asian steppe to integrate the roles of both biotic and abiotic factors in shaping rodent activity patterns.