Abstract: In this study, the effects of isoosmotic solution of Mg(NO₃)₂ and NaCl on seedling growth, leaf lipid peroxidation, antioxidant enzyme activities, and osmotic adjustment substance accumulation were investigated using three cucumber cultivars with different ecotypes. Then salt tolerance was evaluated by membership function method. The results revealed that under the stress of 60 and 80 mmol·L^-1 Mg(NO₃)₂ solution and its isotonic 90 and 120 mmol·L^-1 NaCl solution, the seedling traits such as height, stem diameter, leaf area, fresh and dry mass of aerial part and underground parts, and antioxidant enzymes activity were obviously decreased with the increasing concentration of Mg(NO₃)₂ and its isotonic NaCl in the three cucumber cultivars. Moreover, the inhibitory effects became more obvious with the increasing concentration of either Mg(NO₃)₂ or NaCl solution. MDA content and membrane lipid peroxidation were enhanced in cucumber seedlings. Among the three cultivars, SJ31-1 changed less than the other two cultivars regarding  the reduced amplitudes of biomass, and activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and the  increased amplitude of MDA. In addition, Mg(NO₃)₂ solution inhibited seedling growth more strongly than isotonic NaCl solution did, such difference was relatively significant with increasing the concentration of solution. The contents of proline, soluble amino acids, and soluble sugars varied depending on the cucumber genotype and salt type. The increased amplitude of proline content was the largest in SJ31-1, and that of soluble sugars was the largest in Lubai 19 when growing under salt treatment. The change of these parameters in Xintaimici was in between. Soluble sugars and soluble proteins were predominant osmolytes unde NaCl stress, whereas proline and soluble proteins were main osmolytes under Mg(NO₃)₂ stress. Comprehensive evaluation showed that salt tolerance of the three cucumber cultivars was in order of SJ31-1 > Xintaimici > Lubai 19.