The 2b protein encoded by Cucumber mosaic virus (CMV) plays an important pathogenicity role in many solanaceous hosts, but mechanism of inducing disease is still unknown. In order to investigate virulence of the 2b protein on Nicotiana glutinosa plants, in terms of chloroplast structure and photosynthesis, a mutant Fny-CMV Delta 2bpro, which cannot express the 2b protein, was achieved by introducing mutant sites in the 2b gene of Fny-CMV. N. glutinosa seedlings were inoculated with wild-type Fny-CMV and the mutant Fny-CMV Delta 2bpro, and were analyzed for symptom expression, chlorophyll content, photosynthetic rate, and ultra-structural alteration of chloroplast. Up to 30 days post inoculation, wild-type Fny-CMV caused symptoms of severe mosaic, leaf deformation, and stunting, reduced photosynthetic rate and chlorophyll content, and altered the ultra-structure and morphological characters of the chloroplasts. However, host seedlings inoculated with the mutant Fny-CMV Delta 2bpro expressed only slight mosaic symptom. Their photosynthetic rates and chlorophyll contents were not significantly different from those of the mock-inoculated plants, and the ultra-structure and morphological characters of their chloroplasts appeared to be normal. The observed low photosynthetic rates and chlorophyll contents were related to the breakage of the chloroplast morphology and ultra-structure. Results of Northern blotting showed that the virulence of 2b protein was associated with high accumulation level of CMV progeny RNAs in systemic leaves. Non-expression of the 2b protein reduced the accumulation levels of its genomic RNAs I and 2. The level of subgenomic RNA4, encoding CP protein, was found to be significantly decreased.