The relative contributions of plant transpiration (T) and soil evaporation to total evapotranspiration (ET) were determined by using stable isotope measurements in a semiarid oak woodland in the rocky mountainous area of north China. Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) was used to obtain continuous atmospheric vapor concentration and its δ¹⁸O value at different height gradients in the stand. Cryogenic vacuum distillation and liquid water isotope analyzer were used to determine the δ¹⁸O value of the branch and the surface soil. The results from 4 sunny days showed that water vapor concentrations at 2 m and 11 m above ground were lower than that on the ground (0.1 m) and higher than that in the background atmosphere (18 m), but there was no significant difference of water vapor concentration between 2 m and 11 m. Whereas the diurnal change of atmospheric water vapor concentration exhibited "V", "U" and "W" pattern on four sunny days, the diurnal change of δ¹⁸O showed a "V-shaped" pattern, and the minimum δ¹⁸O value appeared at the time between 12:00 and 18:00. The atmospheric water vapor δ¹⁸O value was greatest at 0.1 m height above ground, followed by 2 m, 11 m and 18 m height, while the diurnal variations of δ¹⁸O for all sampling heights had similar trends. The δ_E values, which were calculated based on the Craig-Gordon model, were all smaller than δ_s on May 25, June 1, June 6, and June 14, indicating considerable ¹⁸O depletion in the water vapor of evaporation. The isotopic compositions in 10:00 - 14:00 intervals were significantly correlated with the reciprocal of water vapor concentration (P < 0.001) and the R² values were higher than 0.71, which suggested that transpiration rate was faster at that period of a day and satisfied the steady state hypothesis of isotope in plant transpiration. The δ_ET was determined from the Keeling plots and regression analyses showed the intercepts for the three intervals were different. The δ_ET value in the oak woodland increased initially and decreased thereafter. The maximum enrichment of δ_ET appeared during 10:00 -14:00. The isotope partitioning result showed that the diurnal change of the contribution of transpiration to total ET in the oak woodland was a convex pattern. The percent of T/ET increased from the morning, reached maximum values during 10:00 - 14:00 interval, with maximum values of 98.68%、96.93%、93.81% and 91.34% for May 25, June 1, June 6 and June 14, respectively. The contributions of transpiration to total ET in 6:00 -10:00 interval were 80.67%、85.81%、56.94%, and 54.98%, in 14:00-18:00 interval were 81.70%,72.23%,61.72%, and 60.94%, respectively. Although the contributions of T/ET in 6:00 -10:00 and 14:00 - 18:00 intervals were smaller than those in 10:00 -14:00 interval, the averaged percent was still more than 69.38%, indicating that plant transpiration played a major role in the oak woodland ET in the low hilly area of north China. The approach would improve the understanding of water exchange of forestry ecosystem.