Sap flow of Robinia pseudoacacia and Quercus variabilis in the city west mountain area of Beijing City was systematically measured from May to September of 2008 using thermal dissipation sap flow probes,combined with environmental factors.Diurnal variation of sap flow displayed a wide-peaked or double-peaked curve on sunny days and a multiple-peaked curve on overcast and rainy days.In May and June,the averaged sap flow velocities of R.pseudoacacia were 1.58 and 1.08 times slower than those of Q.variabilis,respectively,and in July,August,and September,1.391,1.408,and 1.450 times higher,respectively.In May and June,sap flow velocities of Q.variabilis were significantly faster at upper heights than median and lower heights and from July to September,faster at lower heights than median and upper heights on typical sunny days.During the season of rapid growth,sap flow velocities of both tree species were positively correlated with solar radiation intensity,atmosphere water potential,and vapor pressure deficit and negatively correlated with soil water potential.Multiple linear regression equations for environmental factors with sap flow velocity were established,which all met the significance at 0.01 levels with F test.