Abstract: A naturally mutant plant highly resistant to Potato virus Y-vein necrosis strain(PVY^N) has been screened from cv.K326 with virus sap rub inoculation and numbered as SN02.The contents of defensive enzymes in tobacco seedlings and chlorophyll a and chlorophyll b in,and resistances of SN02 and K326 inoculated with PVY^N were determined.The results showed that SN02 was of no disease occurrence after inoculating,while the morbidity of K326 reached 100% after inoculating,and its disease index was up to 84.31.The activities of phenylalanine ammonialyase(PAL) in SN02 and K326 changed in similar way and peaked in the 4th day after inoculation,however,the PAL activity in SN02 were 1.35 times as large as that in K326.The activities of peroxidase(POD),polyphenol oxidase(PPO) and catalase(CAT) in SN02 were higher than those in K326,though showed the same change trends as those in K326.The peak values of POD,PPO and CAT occurred 8,12 and 16 days after inoculation,respectively.The contents of chlorophyll a and b in SN02 were higher than those in K326 in tested time range.