The stem sap flow determining system "ICT2000TE" was used to study the characteristics of stem sap flow of 11yearold Pinus taeda with the spacing of 1 020 plants/hm2 in the autumn and the correlation to environmental factors.The results show that the stem sap flow behaves as a singlepeaked curve:it increases sharply after the sun rises and reaches its peak around noon,and the sap flows weakly or almost stops from sunset to sunrise.The average flow speed is 26 cm/h from 10∶00～17∶00 with the maximum of roughly 54 cm/h.The stem sap of trees with different DBH is significantly different.The 4 sample trees with DBH of 10.2,13.5,15.7 and 16.6 cm consume 13.27,13.28,36.33 and 49.8 kg of water respectively per day.The sequence of the environmental factors influencing the stem sap flow is:air temperature>relative humidity>effective photosynthetic radiation>soil temperature>soil moisture.The equation set up with stepwise regression shows it reaches an extremely significant level.