Through three-dimensional observation and numerical analysis of evapotranspiration characteristics and cool island response of individual urban trees, the cause of vegetation cold island effect was analyzed quantitatively. The results show that 1) the absolute value of cool island intensity in various parts of Ficus microcarpa was basically above 3.0°C, while at surface front (SF), the temperature difference could reach the maximum of ?5.19°C and surface top (ST) can reach to ?3.57°C. 2) The evapotranspiration rate of leaves in the east, SF and SR (surface right), was start to rise firstly, and the leaves in the west, SL (surface left) and SBa (surface back), have the highest evapotranspiration rate at noon while the leaves in the west (SL, SBa) reached the peak later. The transpiration of all parts at night was very weak, less 0.05 mm/h. 3) The overall cool island effect intensity were positively correlated with evapotranspiration rate (pearson correlation coefficient was 0.70, with a significance lower than 0.01). For every 1 mm/h increase in evapotranspiration, the temperature of the whole tree decreased by 3.56°C. Evapotranspiration of surface top contribute obviously to the cool island effect of surface top itself with a correlation coefficient ?0.61. Evapotranspiration of surface top, surface front and surface right have the largest contribution to cool island effect of the whole tree (correlation coefficient absolute value is greater than 0.60). Contribution order of different parts’ evapotranspiration to overall cool island effect is: SF>ST>SR>SL> SBa>SBo (surface bottom), that means east and top side>west side>bottom.