Abstract: A toxic substance produced by Botryodiplodia theobromae in liquid culture, its physical and chemical properties and effects on Cinnamomun cassia were studied. Bioassay indicated that the metabolites of B. theobromae caused wilt on C. cassia new sprouts and necrosis on leaf discs. The toxin is non host specific and toxic to plants other than C. cassia such as Cinnamomum camphora, Pinus massorian, Wedelia prostuata, Lycopersicon esculentum and Mikania micrantha. The toxin was weakly acidic, heat and cold stable, dissolved easily in water, ethanol, acetone, n butanol+acetone, moderately dissolved in acetic ester, slightly in chloroform and not in ether. The toxin did not belong to proteins, nucleic acids and polysaccharides. However, the toxic filtrate contained aldose. The toxicity was increased after ethanol and acetone treatment. High performance liquid chromatography from ethyl acetate extract of the toxin revealed four major fractions. After treated with the toxin, the relative electrical conductivity of C. cassia leaf discs increased dramatically in comparison to that of the control. When exposed to the same level of toxin concentration, the electrical conductivity of susceptible C. cassia is higher than that of resistant C. cassia var. macrophyllumone.