Abstract: To effectively control virus disease at tobacco seedling stage, pot experiments were conducted to study the effects of seed transmitted virus on the occurrence of virus disease. Vigorous tobacco seeds were cultured in a floating system after soaking in solutions of TMV, CMV and PVY at different concentrations. Virus carrying rates and contents of tobacco seedlings were measured at four true leaves stage. The results showed that the seeds soaked in TMV, CMV and PVY solutions were infected with virus disease to different extents. The lowest (threshold) virus concentrations of TMV, CMV and PVY were 7.46×10⁴, 7.44×10⁴ and 7.52×10⁴ copies/μL, respectively. There was no correlation between the amount of virus carried by tobacco seedling and the virus content in solution. Tobacco seedlings infected virus disease when the amount of virus carried by tobacco seeds was higher than the threshold.