Abstract: To partition evapotranspiration into its components, dynamics of water vapor δ18O at five different heights were monitored continuously in maize field by stable water vapor isotopes analysis system combined with Keeling plot curve, which were compared with the estimations through the eddy covariance system and the micro-lysimeter. The results showed that the stable isotope composition of atmospheric water vapor (δv) in maize field was significantly decreased after rainfalls, and linearly related to vapor pressure deficit (VPD) and solar radiation (Rs) with correlation coefficients (R) of 0.65 and 0.51, respectively. Water vapor stable isotope composition (δE) from soil evaporation has significant isotopic fractionation effects near the soil surface. Low soil water content and atmosphere relative humidity alleviated the isotope fractionation. Compared with the results estimated by the EC-MLS method, the error estimated by IS method was less, about -0.02 - 0.08. The results indicated that the combination of Keeling plot method with in situ continuous measurements of water vapor stable isotope composition accurately separated evapotranspiration in maize field. During the measurement period, i.e., after maize canopy moderately covered the soil, the average ratios of crop transpiration to evapotranspiration determined by IS and EC-MLS methods were 0.81and 0.78, respectively.