H₂ O₂ has been suggested and applied as effective algaecide for harmful cyanobacterial bloom control, however, the transport of exogenous H₂ O₂ into microalgal cells, the subsequent intracellular damage pathway and dose-response variations were little studied. We addressed these questions in a bloom-forming cyanobacterium Microcystis aeruginosa with H₂ O₂ at 0.1-1.5 mM. The results showed that H₂ O₂ at 0.4 mM and above significantly suppressed M. aeruginosa growth for over two weeks, and induced apoptosis-like death in terms of membrane potential dissipation, caspase-3 activation, chromatin condensation, and lysis induction. However, the dose-response effects were not monotonic. H₂ O₂ at 0.7 mM resulted in the severest growth suppression among 0.1-1.5 mM treatments, including the lowest biomass for 74% loss, the highest cell lysis ratio for 79%, and the highest utilization rate of H₂ O₂ for 0.101 mM d^-1 . Moreover, several evidence point to severer apoptosis-like cell death in 0.7 mM treatments, involving fastest and severest cell lysis, smallest cell size and wrinkled surface and lowest membrane potential. Therefore, the apoptosis-like cell death induced by H₂ O₂ at moderate dosages should be a crucial cause for the non-monotonic dose-response effects on growth suppression. Additionally, intracellular H₂ O₂ level increased rapidly within 20 min after exposure at 0.4 mM and above, directly confirming the transport of exogenous H₂ O₂ into M. aeruginosa cells and the intracellular damages due to subsequent elevation in intracellular oxidative stress. The study demonstrates that H₂ O₂ at moderate dosages could be a promising method for the biomass control, in a fast and efficient way, on M. aeruginosa blooms.