Abstract: The rates of photosynthesis and leaf transpiration, water use efficiency of two rice (Oryzasativa L.) cultivars of Aixiangnuo and Annong-s grown in open-top chambers with ambient and ambient plus 300mg.kg^-1CO₂ from transplanting to harvest were studied. The results showed that photosynthetic rate and stomatal resistance were increased, while transpiration rate was decreased and water use efficiency improved in elevated CO₂ concentration (660mg.kg^-1). There were difference between the two cultivars' responses to the CO₂ enrichment.