Abstract: The experimental results showed that after rice seedlings were infected by Xanthomonas Campestris pv.Oryzicola, the decrease rate of chlorophyll content in infected leaves of resistant varieties was lower than that of susceptible varieties.During the period from the 2nd to 4th day after inoculation with the pathogen, the activity of Superoxide Disrnutase(SOD).Peroxidase(POD)and Catalase(CAT) in infected leaves of resistant varieties increased significantly,while the activity of susceptible va-rieties SOD and POD decreased,Although the POD activity of susceptible varieties increased,the in-crease rate of that was lower than that of resistant varieties, There was significantly positive relation be-tween the change of SOD activity and the change of POD and CAT activity in resistant varieties,where as susceptible varieties only displayed weak positive relation between therm.So it is suggested that violat-ing original balance among SOD, POD and CAT activity may be a physiological reason that makes sus-ceptible varieties susceptible to the pathogens,