Sap flow of Eucalyptus urophylla×E.grandis in Guangdong Province was measured from July to August in 2010using thermal dissipation probe method.The relations of sap flow to ambient environmental factors were analyzed in combination with environmental factors collected from the micro-meteorological station.Daily sap flow had significant circadian variation with weak sap flow activities at night.The velocity in the north was higher than the south and the correlation between them was significant.There were marked correlations between sap flow and ambient environmental factors〔photosynthetically active radiation(PAR),relative humidity(RH)and wind speed〕,and the correlated degree was in the order of PAR> RH > wind speed.The equation set up with stepwise regression shows that it reaches an extremely significant level.Sap flow at night was mainly associated with water storage in the trunk and had no significant correlation with ambient environmental factors.