Abstract: To reveal the pathogenic mechanism of tobacco target spot disease (Rhizoctonia solani, R. solani), the toxin of R. solani was isolated and purified, method of determining the biological activity of the toxin and the physical and chemical properties of the toxin were studied. The results showed that the toxin was obtainable by isolation and purification, it could damage detached leaf, inhibit seed germination and radicel growth and cause seedling wilt. Puncturing inoculation on detached leaf was a more suitable method for determining the biological activity of the toxin. The toxin is tolerant to heat and light exposure and relatively storable, it is a polar compound, and its active substance is nonprotein.