Abstract: The bacterial strain DH-e， showing strong algicidal activity against Prorocentrum donghaiense， was isolated from the coastal area of Zhejiang province where the harmful algal blooms occur frequently. We probed lytic character and effects of physiological functions and cell structural integrity against P. donghaiense. We found that algicidal bacterium DHe functioned indirectly on the alga cells. Furthmore， the algicidal activity against P. donghaiense was both concentrationand timedependent. When the concentration and the rate reached 0.5%～2.0% and 80% after 32 hours， respectively， it had shortterm and efficient algicidal effect. The molecular weight of algicidal compound was <2 kDa. And it tolerated high/low temperature and was stable in acidic and alkaline conditions. The change in carbohydrate content and chlorophyll a destruction and decomposition show that metabolite significantly reduced physiological metabolic activity of algal cells. SEM and AO/EB double staining showed that algicidal activity had destroyed the membrane integrity and changed the internal structure of algal cells significantly， resulting in large release of cell contents．