Eucommia ulmoides is a tree species commonly used for reforestation in Southern China. It is known to play an important role in water conservation, but the effects of tree structural and environmental factors on the transpiration and water balance of this species have not been fully studied. The aim of this study was to evaluate the diurnal and monthly variability of stem sap flow velocity in E.ulmoides, as well as to understand how structural features and environmental factors were potentially controlling the whole-plant water-use of the species. The study was conducted from July to October in 2004 at the Ecological Benefit Monitoring Station of the Yangtze River Protection Forest in Cili County, Hunan Province (29°30′N, 110°10′E). Stem sap flow velocity was measured using a thermal dissipation probe (TDP, Dynamax, Houston TX, U.S.A). With this probe, data were recorded automatically every 10 minutes. During the same time period, several environmental variables (i.e., mainly meteorological observations) were measured every 5 minutes at a weather station located within the study area. From July to October, stem sap flow velocity of E.ulmoides increased monotonically, reaching maximum values (i.e., 1.818 g/(cm²•h) during October. Diurnal variation of stem sap flow velocity was similar among the different months studied, exhibiting a unimodal pattern that resembled the ones observed for air temperature (T) and solar radiation (RAD). However, there are differences in sap flow starting time and peak time among different months. Stem sap flow velocity was significantly related with solar radiation (RAD), relative humidity (RH), air temperature (T), and wind speed (WS). Among these, RAD and RH seemed to be the most important factors, as was indicated by their high partial correlation coefficients. RAD was the most important factor from July to September, while RH was the most important factor in October. Stem sap flow velocity of E.ulmoides also showed significant correlations with structural characteristics of the forest such as diameter at breast height (DBH), canopy height (Hcanopy), and their interaction of (DBH)2 and tree height (H) (i.e., (DBH)²H).