In the present study, the effect of ozone (O-3) as preoxidant for algae-laden source water, and the mechanisms that it causes algal cells broken and intracelluar cytoplasm release with its influence on microcystins (MC-LR. MC-RR) removal were investigated. Results showed that ozonation of the synthetic algae suspensions prepared from lab-cultured cyanobacteria Microcystis aeruginosa (M. aeruginosa) exerted a significant effect on living cell removal and its growth inhibition under appropriate ozone dosage (3 mg L-1). Synchronously, the algal cell membrane and wall were possibly attacked by the ozonation treatment, which might lead to destroy the cell membrane, release intracellular cytoplasm, increase the dissolved organic carbon (DOC), and finally release the volatile organic compounds and microcystins. Ozonation was found as an effective method in removing of microcystins produced by M. aeruginosa,and the removal efficiency of MC-LR was higher than that of MC-RR. The comparative study of microcystins degradation by ozone in algae suspensions and supernatants showed that the degradation efficiency would be obviously influenced by the algae in solution. The DOC caused organic substances released form algal cell which could consume a significant mount of O-3 would answer for the decrease of microcystins degradation efficiency. (C) 2008 Elsevier B.V. All rights reserved.