Dietary niche separation is often crucial for promoting coexistence of sympatric species sharing similar habitats. In this study, we used cafeteria experiments to explore feeding preferences of two sympatric small herbivores in Hulunber meadow steppe, the narrow-headed vole (Microtus gregalis) and the Daurian pika (Ochotona dauurica), and used microhistological analysis of stomach contents to measure their actual diets and evaluate their niche overlap. To evaluate the effects of scale, we conducted dietary analyses at two commonly documented taxonomic scales of food type categorization: a fine scale based on the species level, and a coarse scale based on a roughly family-level categorization. Niche analysis at the fine and the coarse scales generated qualitatively similar results, although niche overlap was larger at the coarse scale than at the fine scale. In cafeteria experiments, voles and pikas showed similar feeding preference patterns. When foraging in the field, the two species differed in food use and feeding preference, without a significant dietary niche overlap. In the laboratory, both species preferred to eat Leguminosae and Asteraceae species. In the field, pikas still preferred Leguminosae species and voles switched to prefer Liliaceae species. These results suggest the existence of dietary niche partitioning, and competition might have driven the observed niche shift.