The yak (Bos grunniens), sometimes called "the boat of the plateau", is an endemic species of the Qinghai-Tibet Plateau, famous for its resistance to cold and importance as a form of transport. The total number of adults may be close to 10,000 because the total population was estimated to be around 15,000 in 1995. Nevertheless, the species was listed as "Vulnerable" in the World Conservation Union's Red List of Threatened Animals because it is inferred that it has declined by over 30% over the last 30 years. Poaching, including commercial poaching for meat, has been the most serious threat to wild yaks. The species was listed on CITES Appendix I. Wild yaks have been protected in China since 1962, and are currently listed as a Class I protected animal, which means that they are fully protected by government law. In China, wild yaks are found in a number of large nature reserves, including the Arjin Shan, Chang Tang, Kekexili, Sanjiangyuan, and Yanchiwan Nature Reserves, although none of these reserves provide complete protection from habitat loss or occasional poaching. The yak population in the Helan Mountains is a reintroduced population from Qinghai-Tibet Plateau, and is only distributed in the Halawu Valleys and the surrounding areas, in the Helan Mountain Reserve in Inner Mongolia. Because their habitat selection had not been previously studied, we conducted surveys from December 2009 to January 2010 and from April to May 2010 to study yak habitat selection in winter and spring. Twenty line transects were established along seven valleys. We identified a total number of 103 used plots in winter and 129 in spring. To provide comparison plots for the analysis of habitat selections, we surveyed 188 randomly placed plots in winter and 207 randomly placed plots in spring by systematically placing transects in areas with no obvious evidence of yak use. Data on 18 topographic and biological variables were collected in each plot. In winter, yaks preferred montane coniferous forest, lower altitude (<2000 m), habitat dominated by Ulmus glaucescens, more gentle slopes (<10°), sites closer to the foot of the mountain, areas distant from water resource (>1200 m), moderate distance from human disturbance (2000-4000 m) and abundant hiding cover (>70%). In spring, yak preferred subalpine shrubland and meadow, higher altitude (>3000 m), lower tree density (<1 tree/100 m²), lower tree height (<3 m), greater distances from trees (>3 m), higher shrub density (>40 trees/100 m²), short distance to shrub (<1 m) and higher vegetation coverage (>70%). Habitat selection of the yak showed significant differences in vegetation type, landform feature, dominant tree species, shrub type, position on mountain (e.g. top or base), slope direction, distance from human disturbance and distance from water resource between winter and spring. Results of principal components analysis indicated that the first principal component axis accounted for 21.10% of the total variance in habitat use during winter, with the highest correlation coefficient for distance to nearest tree, dominant tree species, tree height, tree density. In spring, the first principal components axis accounted for 31.247% of the variance, with highest correlation coefficient for vegetation type, altitude, landform feature, shrub density. Compared to other migrating yak populations distributed in Qinghai-Tibet Plateau, there were significant differences in yak habitat selection between different seasons in the Helan Mountains. Yaks are acclimatized to the particular geography and climate in the Helan Mountains.