Abstract: Alnus formosana,originated in Taiwan Province,is an important pioneer afforestation tree in Taiwan and can fertilize forestry soil.Water is one of the most important factors to restrict the geographical distribution of plant and to influence introduction of tree.The water ecological characteristics of two-years-old seedlings of introduced A.formosana were studied by using Li-6400 photosynthetic system.The main results showed as follows:(1) The diurnal change of transpiration rate(Tr) of leaves during the whole growth period showed a sort of wide and one-peak curve type,and the peak value occurred at about between 13∶00 and 15∶00,and leaves at different positions had different peak value during various seasons.A.formosana is of non midday depression of transpiration.The daily change of stomatal conductance have both one-peak type curve and two-peak type curve;(2) The seasonal change of Tr of various leaves all showed the double-peak curve, and the maximum of Tr during the whole growth period appeared in July, and the diurnal average Tr of leave is 3.25 mmol·m^-2·s^-1.This tree showed physiological characteristics of both high photosynthesis and strong transpiration;(3) The daily change of water use efficiency(WUE),which was similar to that of Pn, had obvious correlation with that of conductance, and had both the one-peak curve and the double-peak curve.The WUE of June and July were lowest during the whole growing season, and the WUE of August highest;(4) Both the correlative (ana-)(lysis) and the path coefficient analysis all indicated that the air temperature (T_air) or leaf temperature(T_leaf),photosynthetic active radiation (PAR),conductance (Cond) and the relative humidity (RH) are the four most important direct factors to influence the seasonal changes of WUE.