Abstract: A hydroponic experiment was conducted to investigate the effects of 24-epibrassinolide (EBR) on the leaf mitochondria and chloroplast ultrastructure and photosynthesis of cucumber seedlings under hypoxia stress. Under the stress, the apparent quantum yield (AQY) and carboxylation efficiency (CE) decreased significantly, while the light compensation point (LCP), dark breathing rate (R_d), and CO₂ compensation point (CCP) had a significant increase. The application of EBR increased the CE significantly by 29.4%, and decreased the LCP and R_d significantly by 15.0% and 14.4%, respectively. The light response  curve (P_nPPFD) and CO₂ response curve (P_n-C_i) indicated that under hypoxia stress, the increment of net photosynthetic rate (P_n) decreased, while EBR addition enhanced the increment of the P_n. Hypoxia stress damaged the mitochondria and chloroplast ultrastructure, while EBR could alleviate  the damages in chloroplast and mitochondria under hypoxia stress, and keep the leaves in a higher photosynthetic performance.