Abstract: Pythium was isolated from tobacco stems back rot samples in Song County of Henan Province. In order to know its classification and pathogenicity to tobacco, the morphologic characters that based on microscopy technology were examined and recorded, the rDNA- ITS sequences were analyzed, the pathogenicity tests were conducted. The results showed that the colonies were actinomorphic and gradually grow into petaliform; hyphas were without separate; sporangiums were spherical or pear-shaped; archegoniums were spherical; antheridiums are bell-shaped or irregular. According to the results of the analysis of rDNAITS sequences, the sequence homology was more than 99% when it was compared with the sequence of registration number kc014615.1 and gu133597 in GenBank. Therefore, the pathogen was identified asPythium vexans de Bary. Seedings in ten leaf stage of‘Zhongyan 100’tobacco variety were inoculated withP. vexans de Bary, and the disease symptoms and the disease indexes were surveyed and recorded in the 7th day, the disease index ofPythium vexans was 48 and theP. aphanidermatum was 62.7, the results indicated that the pathogenity ofPythium vexans was less thanP. aphanidermatum.