In order to evaluate inactivation effects of steam to Tobacco mosaic virus (TMV) in residue of float tray, The inactivation effects to TMV in dry roots of tobacco float seedling were evaluated by local lesion assay and inoculation to tobacco variety K326. 96℃ steam keep 3 minutes to 60 minutes could highly inactivated TMV in dry roots of tobacco float seedling, which inactivation ratio are 99.9% to 100%. TAS-ELISA and TMV-Imunostrip test show that steam could fully destroy TMV antigen in dry tobacco seedling roots. 96℃ steam keep not less than 3 minutes is recommend to apply in used float tray for TMV disinfection. But foamed polystyrene plastic trays became distortion after treated at 96℃ steam for 5 minutes.