In the mountains, differences in the use of vegetation by animals in higher elevation may reflect their adjustments to harsh environments. Yakushima Island has a mountain with an elevation of 1,936m above sea level and its vegetation changes vertically. Above the forest vegetation, the summit area of Yakushima Island, at more than 1,700m a.s.l., is the sasa grassland, mostly covered with a species of sasa bamboo (Pseudosasa owatarii). We studied the use of vegetation at the highest area of Yakushima Island by two mammal species: Japanese macaques (Macaca fuscata yakui) and sika deer (Cervus nippon yakushimae). This study was conducted from August 2015 to November 2016. We monitored the phenology of some parts of the sasa bamboos and collected location data and food items of two mammal species. We found that mature leaves of sasa bamboos existed throughout all seasons, while buds and shoots of sasa bamboos were present from spring to autumn. Japanese macaques seemed to exhibit seasonal migration to the area; they used the area from spring to autumn to feed on sasa bamboos only when they were available. However, contrary to the monkeys, sika deer seemed to use the area throughout all seasons to feed on both immature and mature leaves of sasa bamboos. Generally, sika deer, which are ruminants, can eat more fiber-rich foods than Japanese macaques, which are monogastric animals. Both species’ distinct digestive systems may contribute to their different seasonal migration patterns to the high altitude sasa grassland.