Abstract: A hydroponic experiment was conducted to investigate the effects of exogenous 24-epibrassinolide（EBR）on the mitochondria antioxidant system and cellular ultrastructure in seedlingsroots of cucumber（Cucumis sativus L.‘Zhongnong 6’）under hypoxic stress. Under the stress，the superoxide anion production rate，contents of hydrogen peroxide（H2O2）and malondialdehyde（MDA）were significantly enhanced，while the root activity was decreased，and the root tip cells were damaged. Compared with the control，the activities of SOD，POD and CAT were induced and were higher at 3 d，
but significantly decreased at 6 d by hypoxic stress. However，EBR treatment could keep higher activitiesof SOD，POD and CAT for a longer time，reduce the generation of ，H₂O₂ and MDA contents，increasethe root activity，and release the damage of root tip cells. The results suggested that exogenous EBR could play a positive role in regulating the mitochondria antioxidant system and keeping cellular ultrastructure of cucumber seedling roots，and alleviate the damage of hypoxic stress.

Key words: cucumber, hypoxic stress, 24-epibrassinolide, mitochondria, antioxidant system, ultrastructure