Abstract: Effects of NO sodium nitropprusside (SNP, an exogenous nitric oxide donor) on reactive oxygen species and protective enzyme activities were studied in the leaves of two tomato cultivars, ‘Hufan1480’ and ‘Hufan2496’, under 100 mmol/L NaCl stress. Results showed that nitric oxide not only increased the activities of SOD, POD, CAT and APX, but also increased the concentration of AsA and GSH, and of proline and soluble sugar in leaves. Results also showed decreased MDA concentrations and O2- producing rate under NaCl treatments. The effect of NO treatment in salt-sensitive ‘Hufan2496’ was more effective than that in salt-tolerant ‘Hufan1480’ under salt stress. Results suggested that NO could increase protective enzymatic activities, contents of osmotic adjustment substances and antioxidants under NaCl stress. Thus, NO played an important role in enhancing the resistance to NaCl stress of tomato seedlings and elevated the salt tolerance of plants through keeping low ROS contents.