Abstract: Application of grafting technology to improve crop resistance to diseases has been successfully demonstrated. In order to clarify the changes of major defense-related enzyme activities and MDA content in leaves of grafted tobacco plants（grafted tobacco plants） inoculated Phytophthora nicotianae, and their relationship with the resistance of grafted tobacco plants, the PAL,PPO, SOD, CAT, POD activities and MDA content in grafted tobacco plants and self-rooted seedlings with P-KRK26 resistant to black shin disease and intermediate susceptible variety G279 as rootstocks and susceptible variety Honghua Dajinyuan（HDa）as scions were analyzed by pot. The results showed that PAL, PPO, CAT and POD activities were elevated to different degrees in the leaves of self-rooted scion Honghua Dajinyuan（HDa）, self-rooted rootstocks P-KRK26 and G279, grafted combinations P-KRK26+HDa and G279+HDa after inoculation with P. nicotianae. The increase of defense enzymes in the leaves of grafted combinations P-KRK26+HDa and G279+HDa were mostly in between the self-root scion and self-root rootstock or significantly greater than that of their self-rooted scion and rootstocks. SOD in the leaves of the grafted tobacco plants showed the trend of decreasing and then increasing, and the decrease in SOD at the peak were significantly smaller than that of their self-rooted scions and rootstocks. The content of MDA in the leaves of the various treatments of tobacco were all elevated to varying degrees,and the increase in MDA of the grafted tobacco plants were between that of their self-root scion and self-root rootstock. The increases of PPO and CAT in the grafted tobacco plants were positively correlated with their resistance to P. nicotianae attack,but the resistance of the grafted tobacco plants were not correlated with the increases of PAL, POD, SOD activities and MDA content, suggesting that these indexes may not be the direct factors of disease resistance in grafted tobacco plants.