Granier's sap flow measuring system was applied to monitor sap flow of Acacia mangium plantation, the pioneer community of re-vegetation in South China's hilly land, for four consecutive years (2004-2007). We aimed at understanding water use by the 20-22 year old A. mangium stand that had already passed the peak period of growth. Results showed that the diurnal change pattern of sap flux density (J_s) presented a unimodal curve with distinct differences between wet seasons (WS) and dry seasons (DS). The sap flow in WS was obviously more dynamics than that in DS, and the peak values of J_s (J_smax) during WS were higher than DS. Partial correlation analyses showed that J_s was significantly correlated with PAR or VPD, and such correlation for DS was much more significant than WS. Linear correlation analyses indicated that the combined effects of PAR and VPD on diurnal changes of J_s attained 62.7%-82.7%. For the whole tree and stand transpirations, marked differences between WS and DS were observed in A. mangium, where the transpiration during WS was higher than DS. Partial correlation analyses indicated that there was no direct significant effect of precipitation on stand transpiration, while PAR was significantly correlated with transpiration and hence the most influential environmental factor on stand transpiration. Annual water consumption by transpiration of A. mangium plantation from year 2004 to 2007 was 238.64, 136.39, 217.45 and 273.85mm, which merely accounted for 24.7%, 11.7%, 9.8% and 22.8% of precipitation for each year, respectively. The annual mean transpiration of A. mangium plantation was (243.31±28.49) mm (n=3). Influence of precipitation on stand transpiration did not merely depend on the rainfall intensity, since an increase in rainfall frequency often reduced the whole year’s transpiration. Since the studied A. mangium plantation was in its late stage of growth with a lower canopy leaf area index and transpiration, the current forest stand could not fully and effectively utilize the local resources such as radiation and water. A stand conversion is required to improve the efficiency of forest management and production in late stage of reforestation.