Plant virus coat (CP) gene-mediated protection is one of the best known approaches to protect against virus resistant transgenic plants. Transgenic N. benthamiana plants containing the CP gene of Zucchini green mottle mosaic virus (ZGMMV) were used for the environmental risk assessment of the living modified (LM) plants with plant virus resistance. The most optimal co-infection method of both ZGMMV and CMV (Cucumber mosaic virus) on Non-LM and CP-expressing LM tobacco plants was established and co-infection of CMV and ZGMMV was confirmed by polymerase chain reaction (PCR). To address the effects of LM tobacco plants on the mutation of the virus, in-vitro transcripts of CP and Replicase (Rep) derived from CMV and/or ZGMMV were inoculated onto Non-LM or LM tobacco plants. Mutation frequency of CP and Rep from CMV and ZGMMV was examined through six serial passages in Non-LM and LM tobacco plants. Little actual frequency of mutation was estimated, probably due to the limited number of transgenic plants tested in this study. However, it does not suggest environmental safety of these CP-mediated LM plants. Further study at a larger scale is needed to evaluate the environmental risk associated with the CP-expressing LM plants.