Abstract: The effect of water stress by treating with PEG (MW 6000) solution with different osmotic potential (-0.4MPa, -1.0MPa, -1.6MPa) on the net photosynthesis rate (Pn) of leaves of popular clones are studied in growth culture chamber. As for six clones of natured-drought, they are grouped into 3 fast-and 3 slowergrowing clones according to the significant difference in growth parameter. During 24 hours of various water stress, the Pn is obviously declined. After release from the water stress, the Pn of recovery of two growth types of clones shows that all clones survive in mild (-0.4 MPa), but not in moderate (-1.0MPa) and severe (-1.6 MPa) osmatic stress at the end of two weeks. During stress and also after release of stress, the net photosynthesis rate of fast-growing clones decreased and also recovered faster than those of other slower growing clones, especially a faster-growing clone with water tolerant appearing the slowly dropping and fast recovering. Pn seems to be its significant growth contribution. This result will be helpful to identify the fast and drought tolerance clones in early-selection of poplar breeding.