Experiments were carried on by using four dominant species of Microcystis of Lake Taihu, including Microcystis aeruginosa 905 (toxic), M. Aeruginosa 469 (non toxic), M. flos-aquae 1028 and M. Wesenbergii 929, grazing by Ceriodaphnia cornuta cultured in a modified BG-11 medium (TN=10mg/L, TP=0.4mg/L) with over 12 day tests. No large colony (above 10 cells) formations in M. Aeruginosa 905, M. Aeruginosa 469 and M. Wesenbergii 929 were observed in the controls and treatments except for M. flos-aquae 1028. The cells proportion of large colony of M. flos-aquae to total cells was significant different between controls and treatments, contributing 22% in controls and 53% in treatments. There were significant differences between controls and treatments in M. flos-aquae in density of different units of unicell, two-cells, small colony (3-10 cells) and large colony. Significant differences were found between controls and treatments in the number of cells per large colony of M. flos-aquae from day 6 to day 12 in the experiment. Our results showed that large colony of M. Aeruginosa 905, M. aeruginosa 469 and M. Wesenbergii 929 can not be obtained by grazing in the C. Cornuta, but C. cornuta is one of important factors in enlarging colony formation of M. flos-aquae.