Abstract: Hypoxia stress will weaken the immune function and antioxidant capacity of Eriocheir sinensis. In order to reveal the immune protection and anti-oxidation regulation mechanism of nanometer selenium (nano-Se) on E. sinensis under hypoxia stress, we had fed E. sinensis with different doses of nano-Se (0, 0.1, 0.2, 0.4, 0.8 and 1.6 mg·kg⁻¹) in basic diets for 90 d. After the feeding, we conducted a hypoxia stress test and injected Aeromonas hydrophila under hypoxia stress. The results show that: 1) The mortality of E. sinensis under hypoxia stress for 24 h and that infected with A. hydrophila under hypoxia stress reached 62.45% and 100%, respectively. The levels of hemocyanin and the hemocyte count in crab hemolymph, and the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) in crab tissues increased significantly under hypoxia stress for 12 h (P<0.05), as well as there was a decreasing trend under stress to 24 h. The contents of lactic acid (LD) and malondialdehyde (MDA) continued to rise from 12 to 24 h under hypoxia stress. 2) Appropriate addition amount (0.1−0.4 mg·kg⁻¹) of nano-Se reduced the mortality of E. sinensis significantly and the lethality of A. hydrophila under hypoxia stress (P<0.05), increasing the levels of hemocyanin, the hemocyte count and the activities of antioxidant enzymes (SOD, CAT, GSH-Px) under hypoxia stress significantly, but decreasing the contents of LD and MDA (P<0.05). The addition of 0.8−1.6 mg·kg⁻¹ nano-Se had aggravated hypoxia stress injury. These results indicate that appropriate addition of nano-Se to the diets can improve the decrease of immune response and antioxidant ability of E. sinensis under hypoxia stress, and the optimal dose of nano-Se in basal diets is 0.2 mg·kg⁻¹.