Abstract: Using the Static Chamber/IRGA, water vapor concentrations in tea plantations with different growth durations (3, 9 and 12 years) were measured and compared for diurnal variations in evapotranspiration (ET) before and after pruning. The study aimed to evaluate the water regulating ability of tea trees which is the dominant land use type in Yixing City, west of Tai Lake watershed. The results showed that diurnal variations in ET rate of tea tree tracked uni-peak curve. ET rose from 6:00, reached peak value at 12:00 and dropped rapidly or slowly afterwards. After pruning, tea tree ET rate dropped considerably. Mean ET rates for the 3-year and 9-year tea trees were respectively measured at 6:00, 9:00, 12:00 and 15:00 on April 23 - the day before first pruning. These rates were higher than those measured on May 17 - the day after the first pruning. Daily mean ET rates for 3-year and 9-year tea trees before first pruning were 3.11 mmol·m^-2·s^-1 and 73.74% higher than that after first pruning (1.79 mmol·m^-2·s^-1). Also amplitude decline in ET for the 3-year tea trees was 36.73% and that in the 9-year tea trees was 48.32%. ET decline range increased with increasing growth duration of the tea trees. The influencing factors of ET were discussed as follows: (1) Although ET trend was similar to that of temperature and tea tree ET closely related with temperature (P＜0.01), air temperature was not obviously different between before and after pruning, 34.46 ℃ on April 23 (the day before first pruning) and 34.30 ℃ on June 14 (the day after pruning). This suggested that air temperature was not the main driving factor of ET in the study area. (2) The study area was in a humid region where soil water content was relatively high but changed with rainfall and tea tree cover. There was no close relationship between ET and soil moisture, a trend different from that noted in semiarid and arid regions. This also suggested that soil humidity was not a main driving factor of ET in the study area. (3) Tea tree height after pruning was almost half of that before pruning, which height change was similar to the change in ET. As a roughness factor of aerodynamics, air near leaf was much influenced by the ambient atmospheric conductance, inducing less boundary layer conductance at lower plant heights. Also as water transport in the soil-plant-atmosphere continuum (SPAC) was inefficient, the tea tree height dwarfing rapidly decreased ET rate. (4) Air temperature was similar and soil moistures relatively higher before than after pruning. As the 20-year tea crop coverage (without pruning) increased by 9.41%, shadow area widened and soil evaporation weakened. While vegetation coverage of the 3-year and 9-year tea trees (with pruning) decreased respectively by 15.85% and 24.93%, bare soil area didn't increase and soil evaporation became smaller. As pruned debris of tea trees covered soil surface, especially for the 9-year tea trees, soil water retention capacity increased, and at the same time ET rate range decreased.