Abstract:We studied the inhibitory effect of gallic acid on Microcystis aeruginosaand the effect of Gallicacid on ultrastructure of M.aeruginosa. M.aeruginosa was stressed with different concentrations of Gallic acid. With the increase of both concentration and time of Gallic acid, the growth of M.aeruginosa was significantly reduced. We observed cellular ultrastructure by SEM and TEM.The cells ultrastructure of M.aeruginosa after 72 h showed the changes, compared with the control, the cells walls of M.aeruginosa became wrinkled, collapsed and ruptured. Cytoplasm thylakoids photosynthetic lamellae showed dispersion and fracture. The phycobilisomes attached to the thylakoids decreased or dispersed to the cytoplasm, the number of the lipid particles and the cyanophycin particles of cytoplasm emerged different degrees increasing, a large void appeared in the nuclear area until most of the algal cells died.Therefore, gallic acid with obvious allelopathy can effectively inhibit the growth of M.aeruginosa.