The plant water use efficiency is not only crucial for plant to maintain a relatively high growth rate but also an important metric of water balance in forest ecosystem. Recent years, sap flow measurement has been widely used as a basis for assessing the water exchange between tree crown and ambient atmosphere for it can keep trees in natural condition without damaging the growth of trees. Many studies on trees' sap flow in single species plantations have been carried out across China, but the studies in natural forests are still few. We measured sap flow of four dominant tree species (Pinus manssoniana, Schima suerba, Castanopsis chinensis and Machilus kwangtungensis) in a mixed conifer-broadleaf natural forest in the Dinghushan Reserve of South China using the Granier's thermal dissipation probe method. The sap flow was measured daily in July and November, 2010, which represented the wet and dry seasons in the region, respectively. The environmental factors such as photosynthetically active radiation (PAR), air temperature (T), relative air humidity (RH) and soil moisture were also recorded. The aims of this study were (1) to investigate the characteristics of sap flow of four dominant tree species, (2) to estimate their relationships with the driving factors of transpiration, (3) and to compare the transpiration rates of different tree species calculated from sap flux density combing with trees' morphological characteristics. The results showed that sapwood area of the four dominant tree species had a significantly (P < 0.05) positive relationship with diameter at breast height (DBH). The coefficient of determination R² of four trees species were 0.99、0.96、0.93、0.81 respectively. Sap flux density displayed an obvious pattern with higher values during daytime and also showed a significant seasonal variation with higher value in wet season. The average sap flow velocity of P. manssoniana, S. superba, C. chinensis and M. kwangtungensis were (10.07±1.47), (10.07±1.51), (12.20±1.73), (8.35±1.28) gH₂O m^-2 s^-1 in wet season, and were (7.22±1.33), (6.41±1.15), (9.78±1.64), (7.37±1.34) gH₂O m^-2 s^-1 in dry season. Contrast to previous study in Dinghushan Reserve of South China at an altitude of 50m, our research found that sap flow velocity displayed a significant variation along the gradient of altitude, which was induced by the differentiation of microenvironment and plant morphological characteristics. Partial correlation analysis indicated that photosynthetically active radiation (PAR) and vapor pressure deficiency (VPD) were the two main environmental factors determining the tree transpiration in both seasons. Transpiration of the four dominant tree species was much higher during the wet season than in the dry season. The average daily transpiration rates of P. manssoniana, S. superba, C. chinensis and M. kwangtungensis during the wet season were 29.52, 39.29, 30.40, 9.41 kgH₂O/d, respectively, while those during the dry season the rates were 20.91, 24.84, 24.26 and 8.43 kgH₂O/d, respectively. The order of the average daily transpiration was: S. superba > C. chinensis > P. manssoniana > M. kwangtungensis both in wet and dry seasons. Such difference in daily transpiration among tree species was mainly determined by the sapwood area and biological characteristics.