How and why species range size varies along spatial gradients is fundamental yet controversial topics in biogeography. To advance our understanding on these questions and to provide insight into biological conservation, we assessed the elevational variations in vascular plants range size for different life form and biogeographical affinities, and explored the main drivers underlying above variations in the longest valley in China's Himalayas---the Gyirong Valley. Elevational range sizes of vascular plants were documented by 96 sampling plots along 12 elevational bands of 300-m ranging from 1800 to 5400 m above sea level. We assessed the elevational variations in range size by averaging the range size of all species within each elevational band. We then related range size to climate, disturbance, competition factors and the mid-domain effect, and explored the relative importance of aforementioned factors in explaining the range size variations using the Random Forest model. Total 545 vascular plants were documented by our sampling plots along the elevational gradient. Out of 545 plants, 158, 387, 337 and 112 were woody, herbaceous, temperate and tropical species respectively. Range size of each groups of vascular plants shown uniform increasing trends along the elevational gradient which are in accordance with the prediction of Rapoport's rule. Climate was the main driver for the increasing trends of vascular plants range size in the Gyirong Valley. Climate variability hypothesis and mean climate condition hypothesis were both supported to jointly explain such climate-range size relationship. Our results reinforce previous notion that Rapoport's rule applies to where the influence of climate is most pronounced, and call for close attention to the impact of climate change in order to prevent range contraction and even extinction under global warming.