Abstract: Hydrogen peroxide acts as an algicide and can also rapidly decomposition into oxygen and water. Becausemany bloom-forming cyanobacteria species produce and release microcystin, we used MC-LR to test whether MC-LR was involved in H2O2 induced growth and metabolic changes in Microcystis aeruginosa. The results showed that M. aeruginosa growth was significantly inhibited after exposure to 250 mol/L and 350 mol/L H2O2. The addition of 200 ng/mL MC-LR could increase the cell viability of M. aeruginosa under 250 mol/L H2O2 stress. The inhibition of PSII activity after exposure to 250 mol/L H2O2 was also reduced in the presence of MC-LR. Moreover, the addition of MC-LR decreased ROS accumulation (P0.05) in cells threated with 250 mmol/L H2O2 and reduced SOD activity (P0.05). Hence, MC-LR could increase the fitness of M. aeruginosa under 250 mol/L H2O2 stress. Our findings contributed to the understanding of growth and metabolism of toxic Microcystis under H2O2 stress.