Abstract: The effects of low magnesium (30% Mg) stress on photosynthesis characteristics and antioxidant system in ‘Jinyou 3’ cucumber (Cucumis sativa) seedlings under low temperature (day/night temperature was 12 ℃/8 ℃) were investigated, with Hoagland nutrient solution treatment as the control. The results showed that the 30% Mg treatment showed a significantly lower Mg content in leaves, compared with the control. However, no marked difference in roots between the 30% Mg treatment and the control was found. Low temperature significantly decreased the chlorophyll content, net photosynthetic rate (P_n), stomatal conductance (g_s), and carboxylation efficiency (CE), while increased the intercelluar CO₂ concentration (C_i). The 30% Mg treatment showed significantly lower chlorophyll content, P_n, g_s and CE, compared with the control. No marked change was found in C_i between the 30% Mg treatment and the control. Low magnesium stress resulted in a more serious damage in chloroplast membrane under low temperature, and showed lower chloroplasts, grains and lamellas, while more and larger starch grains in cucumber seedlings. Low magnesium stress also led to an increase in MDA content, while a decrease in activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR). These data suggested that magnesium deficiency caused by its hindered transportation under low temperature was the main reason for the chlorosis of cucumber leaves. The decrease in P_n was mainly caused by the nonstomatal limitation. Low magnesium stress increased the influence of low temperature on P_n, and the resulting decline in P_n was caused by the stomatal limitation.