Atmospheric water vapor was sampled at two heights above an agricultural grassland in the summer of 1997. The cryogenic method was used for water vapor sampling. The deuterium isotope ratio, δD, and the water vapor mixing ratio, Q, were measured for 11 pairs of samples collected on three summer days. Remarkable gradients of δD and Q existed across the two sampling heights at 0.5 m and 3.0 m. The mixing line method was used to derive the deuterium isotope flux ratio, δDF, which characterizes the evapotranspiration mechanisms of the underlying surface. We found that δDF increased from days 97191 and 97193 to day 97196, indicating a continuous depletion of soil water, whereas the mean δDF falls into the range of the deuterium isotope ratio of regional summer precipitation. In addition, δD of the collected water vapor samples also increased progressively over the same period, which lacked of any significant precipitation.