Chinese cabbage cultivars Qinbai No.2 was used as resistant species and 06J31 as susceptible one. They were infectted by (Xanthomonas campetris pv. campestris) YL-17. The dynamic changes of defensive enzymes activity, O₂^-_· and malondial dehyde (MDA) content in the seedlings were studied. The results showed that, O₂^-_· content declined firstly, and then rose sharply, superoxide dismutase (SOD) activity declined gradually, peroxidase (POD) activity and malondial dehyde (MDA) content rose gradually all the time in the two species after infection. However, the changes of catalase (CAT) activity were different. In the resistant cultivar Qinbai No.2, it rose firstly, declined then, and finally rose again. Whereas, in the susceptible cultivar 06J31, it declined firstly and then rose. It demonstrated that more O₂^-_· were bursted after infection, and SOD and POD activities were higher in resistant cultivar, which were helpful to improve the resistance to black rot and membrane lipid oxidation.