Bentazon herbicide is a post-emergent herbicide and a photosynthesis inhibitor in broadleaf weed. However, different parameters of Bean Kautsky curve changed significantly. Seeds were treated magnetically at three levels normal, 100 mT and 150 mT (3 hours) and weed control (weeding by hand), weed free (untreated), control by 25 percent herbicide, or 0.6 l.ha -1 , 50 percent herbicide or 1.2 l.ha -1 , and 100 percent herbicide or 2.5 l.ha -1 . Due to the major paramagnetic components of bean seeds, affected the parameter minimum quantum yield of PS Π (Fv/Fm) significantly (p<0/01) as in the treatment of magnetic field 150 mT and 2.5 l.ha -1 herbicide decreased significantly (0.61) although Bentazon is a selective herbicide of bean. However, besides posing damage to photooxidation and center PS Π, 150 mT magnetic fields lengthened the time to reach maximum fluorescence (493.3) and maximized the stress to the parameter Fo and Fo/Fm. In the case that no herbicide was applied, it amounted to the maximum fluorescence at the least time (183.3). Also, in the absence of herbicides (both weeding and weed free), basic quantum efficiency were always normal (0.14 to 0.21); however, by using the herbicides, it exceeded its normal range. Therefore, the selective of the herbicide did not mean the immunity of the plant against the herbicide completely. In general, 100 mT magnetic fields were known to be an effective treatment for plant's exposure to Bentazon herbicides and stronger field had the opposite effect.