Abstract: The activity of POX in flue-cured tobacco leaf was increased obviously after infected by TMV and CMV in earlier stage, and changed with different potassium content in tobacco leaf, and decreased with the continuation infection by mosaic virus.However, the activity of POX in tobacco applied potassium was higher than that of the control.The isoenzymes of POX were changed accordingly, that is, two enzyme stripes of tobacco leaf without application of potassium were disappeared, while two weak stripes were emerged.The content of MDA and permeability of plamsa membrane were increased for infection of mosaic virus, and it could be controlled effectively by applying potassitm.Thus infecting symptom of tobacco leaf was lightened.