To compare the effect of air humidity (leaf-to-air vapour pressure difference, VPD) on net CO, assimilation rate, leaf conductance and water use efficiency for leaves of C_3 and C_4 grass species, rates of photosynthesis and transpiration were measured under different conditions of VPD and irradiance using a climate-controlled assimilation chamber. Four C_3 species and fourteen C_4 species were used as materials. All species measured showed a decrease in leaf conductance with increasing VPD, but the decreasing rate of the C_3 species was larger than that of the C_4 species. This shows that the stomata of C_3 species is more sensitive to VPD as compared with the C_4 species, The CO, assimilation rate of C_3 and C_4 species were also decreased with increasing VPD, but the percentage of the reduction was larger in the C_3 species than in the C_4 species. The response of water use efficiency to VPD was larger in the C_4 species than in the C_3 species, There were a marked positive correlations between leaf conductance and CO, assimilation rate in both type species, but the slope of the regression line of the C_4 species was twice of that of C_3 species at the lower range of leaf conductance (below 300 mmol m^<-2> s^<-1>). The stomata1 frequency of C_3 species was greater than that of C_4 species. The differences in the stomata1 sensitivity between C_3 and C_4 species to VPD were discussed in relation to the differences in the stomata1 frequency.