Abstract: In order to reveal the antibacterial mechanism of paeonol, the effects of paeonol on morphology and ultrastructure of mycelial cell of Rhizoctonia cerealis were studied by scanning electron microscopy(SEM) and transmission electron microscopy(TEM), and the effects of paeonol on contents of soluble protein, reducing sugar and chitosan in mycelia of Rhizoctonia cerealis were determined by spectrophotometry. The results showed that after the mycelium were treated with paeonol 12 h, hyphal tips swelled irregularly and hyphal branches swelled or had constriction, and after 24 h, hyphal tips, hyphal branches or the mycelium collapsed. The structural alterations of hyphae included irregular swelling of the hyphal cell membrane, irregular thickening of the hyphal cell walls, the unknown inclusions appeared in the thickened part, the organelles, such as mitochondrions, vacuoles and fat granule, became disordered, the daughter hyphae formed in the necrotic hyphal cells. The contents of soluble proteins and reducing sugar in the mycelia of Rhizoctonia cerealis remarkably decreased under the influence of paeonol, and their levels were negative correlated to the concentration of paeonol solusion. The content of chitosan remarkably increased, and its level was positive correlated to the concentrations of paeonol solusion.