The Granier’s probes were applied to monitor the sap flow of an 18-year old Acacia mangium forest stand in a typical hilly land of South China. Diurnal courses of sap flux density (Js) of 14 sample trees with different diameter at breast (DBH) showed significant differences across individuals (CV：36.42％～80.80％). The maximum Js ranged from the highest of 80.05 gH₂O m－2 s－1 to the lowest of 11.25 gH2O m^－2 s^－1. Meanwhile the individual variations of Js did not present a significant correlation with tree sizes (p>0.24), suggesting that the Js magnitude be not an intrinsic character associated with tree morphology. However, the tree size would be an important factor affecting the temporal change of Js. Although generally a larger tree tends to have a higher total daily sap flow (Et, kgH2O tree^－1 d^－1), the highest sap flow calculated from basal area (Es, kgH₂O dm^－2 d^－1) was not found in the tree with the largest DBH, but in those with slightly smaller DBH class, suggesting that medium-sized trees utilize their structure more efficiently in regard to water usage. The whole-tree transpiration calculated from Js also varied significantly across the individuals (3.35～72.42kgH2O tree^－1 d^－1), and was positively correlated with tree DBH in a form of power function (p<0.001). The results indicated that tree size, population and community characteristics played important roles in measuring stand transpiration using the sap flow system. In spite of the obvious discrepancy among the examined trees, the whole-tree transpiration rate showed similar changing trends which mainly followed the change of climatic factors. A distinct time lag of sap flow behind the actual canopy transpiration, here refers to the photosynthetically active radiation, was found in A. mangium spanning from 40 to 110 minutes among the 14 sample trees. No significant correlation was found between the time lag and DBH, tree height as well as canopy size (p>0.36). After 20 years growth the A. mangiun forest, though cultivated, not only showed obvious heterogeneity in both structure and size, but also in the functional aspect (whole-tree transpiration) among individuals. The Granier’s sap flow measurement system was proven to be an suitable approach in monitoring whole-tree transpiration as well as the canopy water flux of man-made forest on the hilly lands of South China.