Evapotranspiration (ET) is an important component of ecosystem water and energy balances. Improved understanding of the water and energy fluxes between vegetation and atmosphere is therefore important for prediction of the impact of climate change on terrestrial ecosystem. Plantation plays an important role in regional water cycle and budgets. China has the largest plantation area in the world. It is necessary to study the response of plantation ET to environmental factors in China. In this study, water vapor flux was measured continuously through the eddy covariance technique in a nearly 30-year old aged mixed plantation (Xiaolangdi site; 35°01'N, 112°28'E, 410 m) in the hilly zone of the North China in 2007 and 2008. Meteorological measurements were carried out synchronously. The major tree species are Quercus variabilis, Robinia pseudoacacia and Platycladus orientalis. The diurnal and seasonal variations of ET and the environmental impacts on ET were analyzed. The results showed that it was warmer and drier in 2007 and 2008, compared with the long-term averaged values. ET showed a single peak seasonal variation, which was low in fall and winter and relatively high in spring and summer. The yearly maximum ET appeared in May in both years, and the diurnal peak values occurred at about 13 pm. The averaged annual ET and precipitation were 546.1mm and 354.1mm, respectively. The daily average ET in summer (July-August) and winter (December-January) were 2.19mm/d and 0.44mm/d, respectively. Year with more rainfall (2007) was also with relatively high ET. ET of the growing season in 2007 was higher than that in 2008, which led to the higher annual ET in 2007. Temperature (T) was the main environmental factor driving the diurnal and seasonal dynamics of ET. Water vapor pressure deficit (D) also had significant impacts on the diurnal and seasonal variations of ET. The relationship between photosynthetic active radiation (Q_p) and ET was significant at the daily scale but not at the monthly scale. There was no significant impact of soil water content (W) on ET. The averaged daily W was among the range of 0.16-0.18 m³/m³ for nearly 1/3 days of the two years, during which the averaged daily ET was 1.0mm/d. Annual ET was higher than annual precipitation in both years. Annual precipitation accounted for 90.0%, 62.9% of annual ET in 2007 and 2008, respectively. Soil moisture from deep soil layer was likely to contribute the difference between ET and precipitation. Annual ET of the plantation ecosystem in Xiaolangdi site (broad-leaved plantation ) was lower than that of some conifer plantation with similar latitude and that of some tropical or subtropical forests, and was higher than that of some boreal forests. Two year data were used in this study, which led to the difficulty in discussing the mechanism driving the interannual variation of ET. Impact of other factors reflecting the vegetation growth, such as leaf area index (LAI), will be analyzed in the future. And the response of ET to environmental factors under drought stress will be discussed.