Abstract:The characteristics of photosynthetic and transpiration of three common afforestation species (Robinia pseudoacia, Hippophae rhamnoides, Prunus armeniaca) were studied in order to reveal their adaptability to environment in the Loess Plateau. Results showed that: All the tree species are sun plants. Pmax (Maximum net photosynthetic rate) and LSP (Light saturation point) of Hippophae rhamnoides in different slope aspects were obviously higher than the other two species. Hippophae rhamnoides With low α could not efficiently use the light of low intensity (<200μmol·m-2·s-1), and a high WUE in upper slopes allowed it to appropriate use the limited water resources, Therefore this species was acclimated in upper and sunny slopes. With low LSP and high α, Prunus armeniaca tended to carry out photosynthesis with light of low intensity. It was acclimated in lower or shaded slopes. But the low WUE implied potential a harm to environment. In contrast to Prunus armeniaca, α and Pn of Robinia pseudoacia decreased obviously downslope, while they were almost the same at the upper slope. The WUE exhibited a shape of parabola along with photosynthetic active radiation (I) with a peak between 400 and 800 μmol·m-2·s-1. Leaf water content in Prunus armeniaca was significantly higher than in the others, while the chlorophyll content was relatively low. This is the reason of low level WUE of Prunus armeniaca.