Optical　microscope,　environmental　scanning　electron　microscope　(ESEM)　and　transmission　electron　microscope　(TEM)　were　applied　to　study　the　effect　of　different　intensity　ultrasound　on　Microcystis　cell　morphology.　The　results　showed　that　after　the　action　of　low　intensity　ultrasound　(below　the　cavitation　threshold)　on　Microcystis　cell,　extracellular　surface　became　slightly　rough,　and　gelatinous　sheath　became　uneven.　Fragmented　debris　appeared　among　cells　and　intracellular　thylakoids　and　bubbles　were　damaged.　Low　intensity　ultrasonication　make　photosynthetic　capacity　and　buoyancy　control　ability　of　the　cell　suppressed　or　disappeared,　leading　to　the　inhibition　of　growth.　The　main　mechanism　of　low　intensity　ultrasound　on　Microcystis　cell　was　mechanical　action.　After　ultrasonication　of　high　intensity,　extracellular　surface　was　paler　and　rough.　Gelatinous　sheath　became　more　uneven,　and　there　were　many　fragmented　debris.　Cell　interior　was　black.　All　of　thylakoids　and　bubbles　were　broken,　dissolved　and　disappeared.　Cells　present　as　partial　block　or　homogenized　structure.　Some　algal　cells　appeared　pyknotic,　but　most　cell　were　dead.　The　main　mechanism　of　high　intensity　ultrasound　was　cavitation　effect,　accompanied　by　mechanical　action　at　the　same　time.