Abstract: In this study, measurements were made on the leaf water potential (Ψ_l), stomatal conductance (g_s), transpiration rate, leaf area index, and sapwood area of mature Acacia mangium, aimed to understand the relationships of the leaf hydraulic conductance (K_l) with the leaf water use and photosynthetic characteristics of the A. mangium in wet season (May) and dry season (November). The ratio of sapwood area to leaf area (A_sp/A_cl) of the larger trees with an average height of 20 m and a diameter at breast height (DBH) of 0.26 m was 8.5% higher than that of the smaller trees with an average height of 14.5 m and a DBH of 0.19 m, suggesting that the larger trees had a higher water flux in their leaf xylem, which facilitated the water use of canopy leaf. The analysis on the vulnerability curve of the xylem showed that when the K_l  decreased by 50%, the Ψ_l in wet season and dry season was -1.41  and -1.55 MPa, respectively, and the vulnerability of the xylem cavitation was higher in dry season than in wet season. The K_l peak value in wet season and dry season was 5.5 and 4.5 mmol·m^-2·s-¹·MPa^-1, and the maximum transpiration rate (T_{r max}) was 3.6 and 1.8 mmol·m^-2·s^-1, respectively. Both the K_l and  T_{r max}  were obviously higher in wet season than in dry season. Within a day, the K_l and T_r fluctuated many times, reflecting the reciprocated cycle of the xylem cavitation and refilling. The leaf stomatal closure occurred when the K_l declined over 50% or the Ψ_l reached -1.6 MPa. The g_s would be maintained at a high level till the K_l declined over 50%. The correlation between the hydraulic conductance and photosynthetic rate was more significant in dry season than in wet season. The loss of leaf hydraulic conductance induced by seasonal change could be the causes of the decrease of T_r and CO₂ gas exchange.