Abstract: Sap flow velocity (SFV) of Quercus liaotungensis and Populus davidiana, which are two main tree species of secondary forests in the Loess Plateau area of western Shanxi Pro-vince, was measured using a thermal dissipation probe during the growing season from April to October 2012. The responses of SFV to vapor pressure deficit (VPD), photosynthetically active radiation (PAR), air temperature (T) and soil water content (θ) were investigated. The results showed that the diurnal changes of SFV of Q. liaotungensis and P. davidiana were mainly influenced by VPD and PAR in May and June, whereas VPD and T were the determining meteorological factors for the diurnal changes of SFV in July and August. Besides the meteorological factors, θ also had an important effect on SFV. The increases in θ during rainfall events resulted in the increases in SFV of both the two tree species. The average SFV of Q. liaotungensis after rainfall events was 28.3％, 48.6％, 16.9％ and 11.5％ higher than that before rainfall events in May, June, July and August, respectively. The average SFV of P. davidiana only increased, respectively, 0.6％, 4.5％ and 2.3％ after rainfall events in June, July and August. It showed that Q. liaotungensis had a higher water demand and was more sensitive to soil water condition than P. davidiana, while the latter could mana-ge its water consumption more conservatively after raining. The relationship between SFV and VPD could be approximately expressed using an exponential saturation function. The change of the parameters of the fitted exponential saturation function indicated that the SFV could quickly reach its maximum value as soil moisture increased.