Sap flow and environmental conditions were monitored at two Eucalyptus (Eucalyptus urophylla S.T.Blake) plantations at Hetou and Jijia, located in Leizhou, Zhanjiang, Guangdong Province. It was found that daily sap flux density (SFD) of Eucalyptus was closely related to daily atmospheric vapor pressure deficit (VPD) (R2 = 0.76, P = 0.01 at Hetou and R2 = 0.7021, P = 0.01 at Jijia) at both sites. No significant relationship existed between daily SFD and mean daily air temperature at both sites. Daily SFD varied with wind speed Y = -17585X3 + 15147X2 - 1250.7X + 2278.4 (R2 = 0.68; P = 0.01) at Hetou and Y = -101.67X3 - 1.65X2 - 376.4X + 1914.8 (R2 = 0.40, P = 0.05) at Jijia, where Y was daily SFD, X was daily wind speed. Experimental observations yielded the following data: (1) the critical lower and upper daily VPD threshold were 0 and 2 kPa, within which daily SFD varied from 540+/-70 L/(m2.d) to 4739+/-115 L/(m2.d) at Hetou site, from 397+/-26 L/(m2.d) to 3414+/-191 L/(m2.d) at Jijia site; (2) Diurnal SFDs at Hetou site were much higher under low relative humidity (< 30%) and slightly lower under high relative humidity (> 80%) compared with those at the Jijia site; (3) The upper and lower threshold of daily and diurnal RAD for the optimal water use of E. urophylla plantations were 18+/-2.7 and 2+/-1 MJ/(m2.d), 669 and 0 J/(cm2.h) during the observation period.