Abstract: Piper hispidum and Psychotria rubra were grown under greenhouse. Photosynthetic rates of leaves for P. hispidum and P. rubra which were subjected watering were 6.3 and 9.8μmol·m^-2.s^-1 at environmental CO₂ concentration, saturated light density and leaf temperature of 26℃, respectively. Photosynthetic rate of leaves for P. hispidum decreased by 0.38μmol·m^-2·s^-1 during leaf water potential(ψ)declined by 1 bar at lower leaf/air water vapor gradient (△w ). Photosynthetic rate (PN) decreased as △w increased and the relationship was PN=7.02-0.06△w, r²=0.7. The slope of this relationship between PN and △w, decreased during lowing ψ.For P. rubra, the similar response of PN on decreasing ψ was found, but the response of PN on changing △w was more sensitive than that of P. hispidum. The relationship between PN and △w for P.rubra was PN=11.16-0.1△w, r²=0.65.Leaf conductance (g) for these shruhs were much alike at higher ψ, such as -8 bar or -8.75 bar for each, but the change of g in leaves of P.rubra was greater than that of P. hispidum during increasing △w, Lowing ψ could increase water use efficiency(WUE)for both shrubs. And WUE decreased during increasing △w. The results showed that the response of P. rubra which habited in subtropical monsoonal forest was more sensitive than that of P. hispidum which habited in tropical rain forest.