Feral yak were introduced into the Helan Mountains on the border between Ningxia Hui and Inner Mongolia Autonomous Regions, China, by the Buddhist monks of Tibet about two hundred years ago. The feral yak population was estimated to be around 200 individuals at present. The plant species composition of the diet of feral yak was studied in order to better understand seasonal variation from November to December 2008 and from April to May 2009. We collected 5-10 g fecal pellets of feral yak from each dung piles in Halawu drainages, and then mixed them to form a single, species-specific composite sample. Using this method, we obtained 500 g dung in winter and 498 g in spring during the whole study area. The composition in the diet was determined by fecal microhistological analyses. The results showed that feral yak consumed 20 plant species (genera) belonging to 11 families in winter. The main families in the winter diet were Graminea (59.10%) and Compositae (26.05%). The major plant species of winter diet included Stipa spp. (35.86%), Artemisia frigida (23.96%), Agropyron cristatum (8.28%), Cyperus fuscus (7.60%), Chloris virgata (6.81%) and Phragmites australis (6.58%), accounting for 89.09% of the whole diet. The graminoid category accounted for a large proportion of the feral yak's diet in winter. Shrubs made up the second largest proportion. In spring, the main families in the diet were Graminea (52.76%) and Cyperaceae (18.80%), among which 21 plant species (genera) belong to 11 families. Stipa spp. (22.72%), Cyperus fuscus (18.80%), Agropyron cristatum (13.23%), Setaria viridis (9.93%) and Thalictrum aquilegifolium (6.46%), Artemisia frigida (6.31%) were main components in spring diet, contributing 77.45% of the whole diet. The graminoid category predominated in the diet during spring, accounting for 52.76% of the total diet. Forbs made up 36.98% of the diet. Haplophyllum tragacanthoides and Asterothamnus centrali-asiaticus primarily occurred in feces sampled in winter, while Artemisia sacrorum, Potentilla spp. and Ammopiptanthus mongolicus appeared in feces collected in spring. The feral yak also consumed small amounts of Ranunculaceae and Leguminosae, both in winter or spring. The contribution of several plant species eaten by feral yak was different between in winter and in spring. Chloris virgata and Artemisia frigida decreased to 2.28% and 6.31% from 6.81% and 23.96% respectivelyin spring, while Setaria viridis and Oxytropis spp. increased from 1.57% and 0.94% to 9.93% and 4.43%. Stipa spp. was the prevalent species of all plant species recorded as eaten by feral yak in winter (35.86%) and spring (22.72%). Although food niche width in winter (4.785) was lower than in spring (7.770), the Shannon-Wiener diversity index (-2.848) and Pielou's evenness index (-0.951) were higher in winter than in spring (-3.403、-1.180, respectively). Although feral yak also consumed large amounts of Stipa spp., other plant species consumed by feral yak differed from those of yak on the Tibet Plateau, which may reflect adaptations for geographical range, vegetation, and other factors. The main components in feral yak diet were not similar to sympatric ungulates (blue sheep and red deer) in winter, when shared food resources become limited, and thus feral yak are unlikely to compete with these other ungulates during that season.