Lower leaves of tobacco (Nicotiana tabacum L. cv. Xanthi-nc and cv. NahG) were inoculated (first inoculation) with tobacco mosaic virus (TMV). Systemic acquired resistance (SAR) were induced in the remote (upper) leaves of Xanthi-nc against the second inoculation with TMV. Levels of reactive oxygen species (ROS) such as superoxide (O₂) and hydrogen peroxide (H₂O₂) were reduced in the remote inoculated leaves which exhibited SAR. As a result, few necroses only were produced in the resistant upper leaves of cv. Xanthi-nc. ROS and tissue necrotization in the transgenic cv. NahG (unable to produce SAR) were increased in the upper leaves. As a result of the first inoculation treatment, activities of the antioxidant enzymes catalase (CAT) and dehydroascorbate reductase (DHAR) and gene expression level (using RT-PCR) of CAT, superoxide dismutase (SOD) and DHAR were significantly increased in the remote upper leaves of Xanthi-nc only and not in NahG. Benzothiadiazole (BTH) treatment increased ROS levels and total phenolic contents at 6, 12 and 24 hours after inoculation (hai) followed by significant reduction of ROS levels and tissue necrotization 36 and 48 hai with TMV in the upper leaves either in cvs. Xanthi-nc or NahG. Activities of CAT and DHAR as well as the gene expression level of CAT, SOD and DHAR were increased significantly in the upper inoculated cv. Xanthi-nc and NahG leaves treated with BTH. So, virus suppression and a limited number as well as size of necrotic lesions caused by TMV seem to be correlated with the level of ROS and phenolic contents. The result indicated that BTH induced ROS and phenolics early after inoculation which inhibit virus multiplication and as a mild stress it immunized plants against the pathogen by the elevated levels and activities of the antioxidant enzymes. This might support the application of low concentration of ROS (H₂O₂) to increase the plant resistance against pathogens ROS and antioxidants play a pivotal role in TMV-induced and suppressed cell death response, respectively. One can conclude that BTH has a key role in SAR even in the NahG tobacco (which is unable to produce SAR), thus we recommend giving more attention to apply BTH in several crops either in the greenhouse or in field experiments to protect plants against viral infection.