Abstract: Instantaneous responses of the rate of net photosynthesis ( Pn ),stomata conductance ( CS ),water use efficiency ( WUE ),CO2 compensation point ( Γ ),carboxylation efficiency ( CE ),and transpiration ( TR ) to CO2,were monitored at different temperature (25°,30°,35℃) and humidity (RH 30,50,70%) regimes,using leaves of water cultivated plants of four selected poplar clones: Populus simonii (Clone A),P.euramericana 28/86 (Clone B),P.popularis 44 (Clone C),and P.euromericana 25/86 (Clone D).It was shown that temperature and relative humidity could affect the instantaneous response to CO2 in Pn,Γ and WUE .There were significantly positive correlation between the Pn ,or the WUE ,and intercellular CO2 (Ci ).The CS adn TR ,however,were not significantly related to theCi in most of treatments.It may be inferred that the influences of temperature and relative humidity on the responses were mediated by modifying the biochemical process underlying the photosynthesis.