Abstract: Southern blight was a new tobacco disease found in Sichuan,to clarify its pathogenic characteristics and control it effectively,the hypha of the pathogen was collected from tobacco roots in field,it was isolated, purified and named as B1.By means of morphological observation,18S rRNA sequence analysis and pathogenic test,B1 was identified as Sclerotium rolfsii Sacc,which belongs in subphylum Deuteromycotina,class Hyphomycetes,order Agonomycetales,genus Sclerotium.The results of biological characteristics test showed that the lethal temperature of hypha and sclerotium of B1 was 50℃ for 10 minutes,acidic condition was preferable to hypha growth,the best was pH 5.0.