Abstract: Using the electron microscope and biology method we studied the pathogen transmitting, spread ways and insect vector. The results showed that Mycoplasma-like organisms(MLOs)of the pathogen of tobacco witches broom were in the vein cells of diseased tobacco and the MLOs' numbers were many in some samples. The M LOs were found in the salivary gland cells of tobacco capsid (Cyrtopeltis tenuis Reuter)and tobacco aphid (Myzus persicae Sulzer) which were got at diseased tobaccos. It was also observed the way of the pathogen transmitting in tobacco cells and the process of disease. There were two ways of in the tobacco cells:By way of destroying the cell walls and by way of passing through plasmodesmata, The M LOs could invade into a new healthy cell. The result of biology experiment demonstrated that not only the tobacco capsid but also the tobacco aphid were vehicular insects of tobacco witches broom. The disease could be infected by grafting diseased tissures and plant vector and dodder (Cuscuta chinensis).