To analyze the altitudinal distribution of seed plants in the central area (∼590 km²) of the Qinling Mountains (central China), supporting a giant panda population, we conducted floristic surveys at discrete altitude intervals and reviewed the data in published floras for the area. We found that there are 2031 species of seed plants in the study area, representing 694 genera and 148 families. These seed-plant species account for 59.1% of all those known from the Qinling Mountains and 7.6% of those found in China. The proportion of pantropical species decreases with increasing altitude, and above 2800 m a.s.l. tropical species do not exist. The proportion of northern-temperate plants increases with increasing altitude. The area accommodates predominately a mixture of Sino-Japanese and Sino-Himalayan forest-region species. A regression model predicted a theoretical boundary between the subtropical and warm-temperate zones at 530 m above sea level.