Abstract: The effects of exogenous spermidine (Spd) on reactive oxygen species (ROS) levels and antioxidant enzymes activities in leaves and roots of two cucumber cultivars seedlings, differing in salt stress resistance, were investigated under salt stress in a nutrient solution culture system. The results showed that exogenous Spd decreased O^-.₂ production rate, H₂O₂ and MDA contents, while the activities of antioxidant enzymes, including superoxide dismutase(SOD), peroxidase(POD) and catalase(CAT), were promoted in seedlings under 50 mmol/L NaCl stress. Spd treatment in ‘Jinchun No.2’ seedlings with lower salt-resistance was more effective than that in ‘Changchun mici’seedlings with stronger salt-resistance. It indicates that exogenous Spd treatment could mitigate the injuries caused by NaCl stress through promoting antioxidant enzymes activities and suppressing ROS levels in seedlings under salt stress.