To evaluate the protective effects of exogenous spermidine (Spd) on tomato seedlings against salt stress, antioxidant enzymes activities in roots and leaves, dry weight of seedlings, and the ratio of root to shoot (R/T) were assayed in two tomato cultivars(Cucumis sativus cv.Baiguoqiangeng and cv. Jiangshu NO.14) with different resistances to salt stress after their seeds were pre-socked with Spd and their seedlings were treated with NaCl. Three experimental treatments from different concentrations of Spd and NaCl were used, (a) dd H2O + 0 mmol/L NaCl, as the control, (b) dd H2O + 100 mmol/L NaCl, and (c) 0.25 mmol/L Spd + 100 mmol/L NaCl. The results showed the activities of three antioxidant enzymes of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), production rate of superoxide anion (O?-2), and content of hydrogen peroxide (H2O2) in leaves and roots of tomato seedlings all increased under the salt stress. Dry weight of tomato seedlings reduced significantly, while the ratio of root to shoot(R/T) increased in both cultivars. The treatments of pre-socked seeds with Spd can increase antioxidant enzymes of SOD, POD, and CAT, and decrease contents of O?-2and H2O2 in the seedlings. Pre-socked seeds with Spd had better effects to the salt sensitive cultivar ‘Jiangshu NO.14’ than to the resistant cultivar ‘Baiguoqiangfeng’.These results indicated that pre-socked seeds with Spd can alleviate salt stress injure resulting from an increase of antioxidant enzymes activities and maintenance of l low ROS level in the seedlings.