Abstract: In this study, cucumber seedlings were cultivated in nutrient solution with different concentrations of spermine（Spm）(0.5, 1, 1.5 and 2 mmol/L). The effects of exogenous Spm on growth of cucumber seedlings, activities of antioxidant enzymes and photosynthesis in cucumber leaves under NO-3 stress were investigated. The results show that activities of superoxidase(SOD), peroxidase(POD), catalase(CAT)and ascorbic acid peroxidase(APX)are significantly increased in the treatment of 1 mmol/L Spm for 10 d under 140 mmol/L NO-3 stress, and stomatal conductance(Gs), intercellular CO2 concentration(Ci)and net photosynthetic rate(Pn)in leaves are significantly enhanced, while electrolytic leakage(EL), the accumulation of malondialdehyde(MDA)and stomatal limitation value(Ls)in cucumber seedlings are significantly decreased. These findings suggest exogenous Spm could enhance the capacity of scavenging active oxygen species and improve photosynthesis, protect cucumber seedlings from the peroxidation of membrane lipids, promote the growth and increase resistance to high concentration of NO-3 stress. After the cucumber seedlings growing in the 1.5-2 mmol/L Spm for 10 d, compared to growing in the 1 mmol/L Spm, activities of SOD, POD, APX and CAT, Gs, Ci and Pn are decreased under the 140 mmol/L NO-3 stress, while MDA level and electrolytic leakage are increased, resulting in injury of cucumber seedlings. These results indicate that the effect of exogenous Spm is dependant on the concentration of Spm. Excessive Spm treatment would decrease the resistance to NO-3 stress.