Stomatal behavior under global climate change is a central topic of plant ecophysiological research. Vapor pressure deficit (VPD) and phytohormones can affect stomata of leaves which can affect gas exchange characteristics of plant. The role of VPD in regulating leaf gas exchange of three tree species was investigated in Jinan, China. Experiments were performed in June, August, and October. Levels of three phytohormones (GA 3 , IAA, ABA) in the leaves of the three trees were determined by highperformance liquid chromatography in three seasons. The responses of stomatal conductance (g s ) to an increasing VPD in the leaves of the three trees had peak curves under different seasons, which differed from the prevailing response pattern of g s to VPD in most literature. The peak curve could be fitted with a Log-Normal Model (R 2 = 0.838-0.995). The VPD/RH values of the corresponding maximum of g s (g s-max -VPD/RH) could be calculated by fitted models. The g s-max -RH could be affected by environmental conditions, because of positive correlation between g s-max -RH and the mean monthly temperature in 2010 (R 2 > 0.81). Two typical stomatal models (the Leuning model and the optimal stomatal behavior model) were used to estimate g s values, but they poorly predicted g s in the three trees. The concentration of ABA was positively correlated to sensitivity in response of stomatal conductance to VPD in the leaves of the tree species during the different seasons. ARTICLE HISTORY