Abstract: The atmospheric CO2 concentration is predicted to double to 700 μl·L-1 during this century. The effect of doubled atmospheric CO2 on higher plants has been studied extensively, but little is known for aquatic plants, especially freshwater bloom-algae. In order to study the impacts of elevated atmospheric CO2 concentration on freshwater bloom-algae, Anabena flos-aquae was cultured with doubled CO2 concentration (700 μl·L-1) and its growth, photosynthesis and chlorophyll and fluorescence were investigated. Atmospheric CO2 enrichment increased the biomass by 50% at the end of culture in comparison with the ambient CO2 concentration (350 μl·L-1). Light-saturated photosynthetic rate, photosynthetic efficiency and photochemical efficiency of PSⅡ (Fv/Fm) of Anabena flos-aquae also increased by CO2 enrichment. However, elevated atmospheric CO2 concentration had no significant effect on the dark respiration and light-saturated points. The photosynthetic affinity for Dissolved Inorganic Carbon (DIC) reduced when the cells were grown in CO2 enriched conditions, which implies that HCO-3 utilization was depressed. It was concluded that elevated atmospheric CO2 concentration resulted in the increasing biomass, which is due to the elevated photosynthetic rate.