Abstract: Understanding of the relations between plant and water is the key to successfully implement protective afforestation programs in the Hilly-gully region of the Loess Plateau,West Shanxi Province.Our study measured sap flow of 17 trees covering 4 diameter classes in a 16 years old Pinus tabulaeformis and Robinia pseudoacacia mixed forest stand from July to October in 2008 using Thermal Dissipation Probe(TDP) in Caijiachuan watershed in Jixian County,Shanxi Province.At same time,we also measured the changes of environmental factors.The diurnal fluctuation of sap flow velocity of Pinus tabulaeformis and Robinia pseudoacacia was experienced a significant difference in the starting time,time to peak,dramatic decreasing time,and lasting time duration of sap fow velocity.The diurnal transpiration rate increased against the diameter of breast height except for 6 and 12cm pine trees.We found that the solar radiation and vapor pressure deficit were the major factors to control the canopy transpiration of Pinus tabulaeformis and Robinia pseudoacacia.In addition,monthly stand transpiration scaled up from sapwood area were 30.8mm,24.1mm,26.3mm,and 18.4mm for July,August,September,and October,2008,respectively.The total transpiration amount during this four months period was less than the total rainfall amount during the same time.