Abstract: In semi-arid regions of the Loess Plateau,Zizyphus jujube and Ulmus pumila samples were selected to study the net photosynthetic rate, transpiration rate and stomatal conductance using LI-6400 portable photosynthesis machine under different controlled soil water contents. Net photosynthetic and transpiration rates were observed under controlled solar radiation intensity through shading to determine water use efficiency. The results show obvious differences in the daily processes of net photosynthetic rate between Z. jujube and U. pumila under different soil water contents. The daily range of change in net photosynthetic rate and water use efficiency for Z. jujube is higher than that for U. pumila. Analysis of continuous daily transpiration rates show that soil water is a major influencing factor of evapotranspiration. Z. jujube has a higher water-consumption rate which decreases at high soil moistures. Therefore U. pumila is more suitable to semi-arid conditions of the Loess Plateau regions than Z. jujube .