Abstract: The Hydraulic Limitation Hypothesis predicted that stomata conductance decreases by tree height,consequently decreases carbon gain in leaves and limits the growth of the tree top,but an experiment in Pinus ponderosa demonstrated that non-structural mobile carbon compounds (NSC) did not decrease by tree height.Our objective was to understand whether leaf carbon gain show the same trend in broad-leaved species,and to understand the structural changes in leaves by tree height.We sampled the twigs which were fully sun-exposed at different height of Castanopsis wattii trees and measured the size of leaves,the size of epidermal cells, the concentration of NSC and the diameter of conduits in xylem.The size of leaves and epidermal cells decreased by height,indicating that the growth of leaves in top was limited.Leaf photosynthesis per unit mass and per unit area did not decrease.The concentration of soluble sugar per weight and per area increased by increasing height and the concentration of starch per weight and per area did not change,indicating that the limitation of tree top growth may not due to carbon gain.The diameter of xylem conduits decreased,suggesting that the ability of water conduct decreased by increasing height.Therefore,top leaves of the tree may suffer more negative water potential and lower turgor which limit the cell expansion.