Abstract: By the method of thermal dissipation,the characteristics of stem sap flow of P.×euramericanacv plantation was studied.Environmental factors including air temperature,air relative humidity,photosynthetically active radiation(PAR),soil temperature,wind speed and soil water content were also measured simultaneously with sap flow.The result showed that there were marked differences between sap flux density at south and at north on the trunk and sap flux density was found to be highly correlated between south and north.There existed marked differences in different months,sap flow rate was incresed from May to July.Difference can be seen at different trees,respectively,canopy size is the major influenced factor.There was a time lag between sap flux and PAR?vapor pressure deficit(VPD).The time lag for Par and VPD were ahead 30-60 min and after 120-150 min respectively.PAR was the dominant environmental variables affecting the diural variation of sap flow density in May,June and July,and in August were PAR and VPD.