Populus deltoids is a fast growing and high water-consuming species. It is necessary to choose clones with high water use efficiency when they are introduced to China. In our test, long-term water use efficiency(WUEL), instantaneous water use efficiency(WUEi), foliar carbon isotope composition (δ13C), photosynthesis, stomatal density and stomatal conductance(Gs) of three P. deltoids clones(DN-2, P. deltoides×P. nigra; R-270, P. deltoides×P. nigra; NE-19, P. nigra×(P. deltoides×P. nigra)) were studied under different water treatments. The results showed that significant differences in these parameters were detected among three clones, indicating the variation of water use efficiency among three clones. NE-19 was the best clone, with the highest WUEL, WUEi, δ13 C and net photosynthetic rate(Pn) and the lowest stomatal density and Gs. Therefore, we make a conclusion that stomatal density and conductance are the major factors which could lead to the variation of Pn and WUE i and in turn affect WUEL and δ13C. δ13C could be a good indicator to evaluate the WUEL of clones, which was correlated with WUEL under sufficient water supply; however, the correlation decreased under water stress. ERECTA gene is the first gene which could regulate plant transpiration efficiency in Arabidopsis. A cDNA clone, designated PdERECTA, was isolated from P. deltoids. RT-PCR indicated that PdERECTA gene may have the similar function in P. deltoids.