Abstract: Foliar carbon isotope composition (δ¹³C), long-term water use efficiency (WUE_L), photosynthesis, and stomata of different <Populus deltoids clones were studied under different water treatments. The results showed that the differences in WUE_L, δ¹³C, photosynthesis and stomata among the tested clones were obvious. Under four water treatments, J₂, J₆, J₇, J₈, J₉ were good clones in WUE_L, with higher δ¹³C, relative higher net photosynthetic rate (P_n), instantaneous water use efficiency (WUE_i),optimal stomatal frequency and stomatal size, higher superior/inferior ratio. The stomatal differences might be a key factor, which leading to P_n and WUE_i differences, and ultimately inducing WUEE_L and δ¹³C differences. WUE_L was positively related to δ¹³C, and the best performance occurred under well watered treatment.