The bacterium associated with ratoon stunting disease of sugarcane usually gather in the 2-3 internodes of base stem section of sugarcane. The ultrastructures of the stem tissue infected by ratoon stunting disease were observed with electron microscopy. The results suggested that cytopathological alteration apparently occurred inside cells and on the cell wall. A large amount of electron-dense substances accumulated in the infected cells, which was presumed to be associated resisting, reducing or delaying the invasion of this disease. The electron-densed substances were only found in the infected cells. The cell wall was thickened and there was a layer of adhesive materials in the vessels of vascular bundle. Some cell walls were uneven with different thickness, and parts of them were broken into crumbs and fiber silks.