It was vital to determine 18O value of evapotranspiration components and partition evapotranspiration (ET) into evaporation (E) and transpiration (T) of Platycladus orientalis forest in Beijing mountains area, it could provide important basis for the research of water-vapor exchange through the whole soil-plant-atmosphere continuum of regional area. 18O of atmospheric water vapor was obtained by a continuously water isotope analyze system, at the same time, branch and soil samples were collected and 18O of all samples were determined using a liquid isotope analyzer. Furthermore, we could estimate the oxygen isotopic composition of evapotranspiration vapor (δET) by a Keeling plot, and the oxygen isotopic composition of soil water vapor (δE) and transpiration vapor (δT) were determined by Craig-Gordon equation and the assumption of transpiration vapor had the same isotopic composition as stem water ( V= X), respectively. After obtaining these parameters, we calculated the contribution of transpiration to total evapotranspiration. The results showed that: (1)δE ranged between -20.26‰ and -17.48‰ during the four experimental days, S> V> E. It was concluded that δE depleted markedly in the process of evaporation. (2)δT increased from -7.31‰ to -2.09‰ during the four experimental days, and δET decreased from -3.84‰ to -8.21‰, which was calculated by Keeling plots and regression analysis showed that R2 for all experimental days were higher than 0.89, and E ET X. It showed that in the middle of the day at noon (we chose 12:00～14:00 as our sampling time), steady-state O18 isotopic composition could be satisfied. (3)The result of evapotranspiration partition demonstrated that the percent contribution of transpiration to total evapotranspiration ranged between 85.37% and 93.04%, the research results revealed that transpiration was the main component of total evapotranspiration, and water consumption of tree transpiration was much larger than that of soil evaporation.