Water flux was measured continuously in fast growing season (from June to August) in 2007 and 2008 using the eddy covariance technique in a 30-year-old mixed plantation (Quercus variabilis, Robinia pseudoacacia and Platycladus orientalis) in a hilly area of the North China. The results showed that seasonal variation of decoupling coefficient (Ω) and canopy conductance (g_C) had a similar pattern. Range of Ω in 2007 and 2008 were 0.12-0.62 and 0.08-0.54 respectively. Average values of Ω in these two years were 0.37 and 0.23. Evapotranspiration (ET) was mainly controlled by stomatal movement in fast growing season. It was dryer in 2008 than in 2007, and the averages of Ω, gc and ET in 2008 were also lower than those in 2007. Stomatal control was the main impact factor on ET in drought. The restriction from stomata on ET was higher in lower atmosphere humidity, and the restriction was aggravated in drought. Radiation was the main environmental factor that affects ET in fast growing season, and drought enhanced the control of radiation on ET. Stomatal control was stronger in the year with higher radiation.