Abstract: Cyanobacteria blooms that appear in eutrophic freshwaters around the world are contaminating water seriously. The toxic compound microcystins (MCs), a family of monocyclic hepta-peptide, generated by cyanobacteria blooms have been reported to have harmful effects on human being and domestic animals. In this paper, we collected cyanobacteria blooms from Taihu Lake, China and employed three sublethal dosages of cyanobacteria bloom extract (CBE) (16, 32 and 64 mg lyophilized algae cells/kg body weight, with contents of 4.97, 9.94 and 19.88 μg MCs equivalents/kg body weight) to assay its effects on the antioxidant enzymes in mice's liver. The mice received CBE treatment every day via i. p. injection for 14 days and were sacrificed on day 3, 5, 7, 9, 11, 13 and 15. Three main antiox idant enzymes: superoxide dismutase (SOD), catalase (CAT) and glutathione-Stransferase (GTS) were ex amined. Inhibition effects of CBE on SOD and CAT activities were presented. Obvious changes of GST activity were also observed. To further determine the degree of peroxidation induced by CBE, the level of lipid peroxidation was analyzed by testing malondialdehyde (MDA) concentration. A significant dose-dependent increase of MDA level was found after 14 days of CBE exposure. Our results showed that CBE possessed the deleterious effects on antiox idant enzymes and caused the increase of lipid peroxidation level in mice's liver.