In this paper, Microcystis aeruginosa co-rultivated with Hydrilla verticillata was used to study the inhibitory effect oi Hydrilla verticillata on Microcystic aeruginosa and its mechanism.The results indicated that Microcystis aeruginosa growth was obviously inhibited by Hydrilla verticil-lat.Its ultra-structure was destroyed and finally disintegrated;rhlorophyll-a rontenl, photosvn-thetic rate declined evidentlv from the beginning;whereas respiratory rate and SOD activity increased firstly, hut decreased as treatment time went on;MDA accumulation was consistent with the degrees of its cell destruction.Hydrilla verticillata culture had the same inJiihitory effeol on Microcystic aeruginosa.The crude excretion from Hydrilla verticillata culture whic h had the in-hibitory effect on Mirorystis aeruginosa was obtained.