Abstract: Staphylococcus aureus was treated with antagonistic bacterium X23 fermentation liquid, then the number of bacterial colonies was counted using agar plate pouring method, and the morphology and ultrastructure of the bacteria were observed using scanning electron-microscope (SEM) and transmission electron- microscope (TEM) respectively to study the bactericidal effect and partial mechanism of antagonistic bacterium X23 fermentation liquid on S. aureus. The result showed that the number of S. aureus colonies in the treated groups was significantly lower than that in the control group (P<0.01). In the group treated for 60 min, the surface of S. aureus had many small protuberances and the cell membrane was folded and eventually ruptured, inside the bacterial cell, massive electron penetration areas appeared, contents in cytoplasm became sparse and uneven, intracellular vacuolar degeneration was aggravated. Besides, cell division was reduced, and bacterium cells were dissolved. These results indicated that antagonistic bacterium X23 fermentation liquid could kill S. aureus through disrupting the bacterial structure.