Frequent outbreaks of cvanobacterial blooms in eutrophic water-bodies throughout the whole country, in particular Microcystis, have been a great threat to water resources. Many researches have been carried out in the primary characteristics of Microcystis, among which, the studies on utility of inorganic carbon may help us further understand the effects of inorganic carbon to the formation of Microcystis bloom and possibly provide clues for the control of Microcystis blooms. In this paper, three strains of M. aeruginosa were studied on their responses of photosynthetic oxygen evolution to different extracellular concentration of dissolved inorganic carbon (DIC) under different photosynthetic conditions, respectively. When the temperature rose from 20 degrees C to 30 degrees C the photosynthetic oxygen evolution rate of M. aeruginosa increased. Their photosynthetic oxygen evolution rates were enhanced at high pH value, the maximum of which increased obviously from pH 7.0 to pH 9.0. While extracellular carbonic anhydrase inhibitor acetazolamide (AZ) showed little effect to the photosynthetic oxygen evolution of M. aeruginosa, intracellular and extracellular carbonic anhydrase inhibitor ethoxyzolamide (EZ) suppressed their photosynthetic oxygen evolution. Simulated with enzyme kinetic equation, their K-0.5(DIC) were, found to be less than 50,mu m, suggesting that M. aeruginosa has relatively high affinity for environmental dissolved inorganic carbon(DIC). The change of temperature had little effect on K-0.5 (DIC) of M. aeruginosa indicated that the ability of utilizing inorganic carbon of M. aeruginosa was not influenced by the temperature within certain range. With the rising of the medium pH, K-0.5 (DIC) of M. aeruginosa FACHB905 and M. aeruginosa PCC7806 increased while that of M. aeruginosa FACHB469 decreased. K-0.5 (DIC) of M. aeruginosa did not change when AZ or EZ was added to the medium. The responses of photosynthetic oxygen evolution of M. aeruginosa and Chlamydomonas reinhardtii under conditions of different pH value and addition of carbonic anhydrase inhibitors were also compared in this paper. The results showed that the photosynthetic oxygen evolution of C. reinhardtii decreased with the rising of pH value from 7.0 to 9.0 and was suppressed by both of AZ and EZ. K-0.5 (DIC) of C. reinhardtii also decreased with the rising of the medium pH and under the treatment of AZ or EZ.