Abstract: To investigate the determinants of zooplankton community structure in aquatic ecosystems under cyanobacterial bloom and the effects of Microcystis aeruginosa density on zooplankton competition relationship, the interspecific competition between Brachionus calyciflorus and Daphnia magna was evaluated by conducting experiments at three concentration gradients of M. aeruginosa. Our results showed that the concentrations of M. aeruginosa had significant effects on the growth and competition between B. calyciflorus and D. magna, it did not change the dominance of D. magna. The results also showed that a low level of addition of M. aeruginosa (5×10⁴ cells/mL) promoted D. magna population growth (P<0.01) and increased dominance in D. magna in the mixing culture. The growth of both D. magna and B. calyciflorus was significantly (P<0.01) affected by the medium concentration (1×10⁵ cells/mL) level of M. aeruginosa. However, population growth of both species was suppressed (P<0.01) by the high concentration of M. aeruginosa (5×10⁵ cells/mL) and only D. magna population survived in the co-exist system. Thus, in the absence of other external influences, our results suggested that D. magna dominated under various concentration levels of M. aeruginosa co-exist with B. calyciflorus, indicating that the concentration of M. aeruginosa may not be the main factor affecting the competitive position of D. magna and B. calyciflorus. The study on zooplankton community dynamics under different concentrations of M. aeruginosa will contribute our understanding the ecological consequences of cyanobacterial bloom.