Sap flow in a typical hilly Acacia mangium forest stand of the Southern China was measured continuously using the Granier′s thermal dissipation probe method over a long period. Environmental factors (including air temperature, relative humidity, photosynthetically active radiation, soil moisture) were measured simultaneously. Sap flux density of sample trees representing the whole stand were selected for calculating the individual tree transpiration combining with morphological characters. Photosynthetically active radiation (PAR) was grouped into following six classes: 100-200μmol·m-2·s-1, 200-400μmol·m-2·s-1, 400-600μmol·m-2·s-1, 600-800μmol·m-2·s-1, 800-1000μmol·m-2·s-1, ≥1000μmol·m-2·s-1, and the relationship between whole-tree transpiration (E) and VPD in the wet season were analyzed for each RAR class. Potential transpiration in the dry season was then estimated based on the fitted regressions with VPD of the wet period. The results showed that the transpiration in the dry season was significantly lower than and was comprised only 10%-20% of that in the wet season. Concerning the reduction of transpiration at different diameter-classes, it was ranked as: dominant trees>intermediate trees >suppressed trees. Compared to those in the wet season, the sap flow of different trees was lower as a result of deficit of soil water content in the dry season. Soil water deficit limited transpiration to certain extent, and the growth of A. mangium forest was subjected to water stress during dry period.Key Words: Acacia mangium; dry season; wet season; whole-tree transpiration; diameter-class