The most important food competitor of the critically endangered Przewalski's gazelle (Procapra przewalskii) is the domestic Tibetan sheep (Ovis aries) in the steppe and deserts around Qinghai Lake on the Qinghai-Tibet Plateau. We used microscopic analysis of fecal samples from August 1998 to August 2001 to estimate botanical composition in the diets of Przewalski's gazelle and domestic Tibetan sheep. We found that approximately 63–76% of the diet of the Przewalski's gazelle consisted of graminoids, composites, legumes, sedges, and Rosaceae. These plants made up 71–92% of Tibetan sheep diets. Dietary overlap between Przewalski's gazelle and Tibetan sheep ranged from 61% during the plant-growing period to 81% during the plant-withering period, indicating severe food competition between the domestic sheep and gazelle. The dietary diversity of Przewalski's gazelle ranged from 26.5 to 35.6 and that of Tibetan sheep from 37.7 to 40.1. The diversity of plants in the diets was similar between the Przewalski's gazelle and Tibetan sheep (P > 0.05), using the independent t-test. To protect the gazelle, managers should control the number or distribution of the Tibetan sheep and take actions to increase food production for Przewalski's gazelle.