Abstract: Trachinotus ovatus juveniles (225 individuals) were fed for 58 d with three diets containing 0 (Control: TC), 0.7 (A1) and 6.0 g·kg⁻¹ (A2) alginate oligosaccharide (AO) to investigate the effects of AO on the growth, plasma biochemical and immune indexes, hepatic antioxidative capacity, intestine morphology and expression of genes involved in Nf-κb signaling pathway of T. ovatus. The results show that the weight gain rate and specific growth rate of T. ovatus of both A1 and A2 group were significantly higher than those of TC group (P<0.05). The feed coefficient of A1 group was significantly lower than that of TC group (P<0.05). Compared with TC group, the mass concentration of plasma C3 increased in both A1 and A2 group significantly (P<0.05), and the activity of alkaline phosphatase increased in A2 group significantly (P<0.05). Compared with TC group, the activities of SOD, CAT, POD and GR and T-AOC increased in both A1 and A2 significantly, while the concentration of MDA decreased significantly (P<0.05). The height of villus in A1 and A2 group were significantly higher than those of TC group (P<0.05). Compared with TC group, the expressions of ikk, nf-κb, tnf-α and il-8 decreased in A1 and A2 group significantly, while the expressions of tgf-β increased significantly (P<0.05). In conclusion, AO supplementation in diet can improve the growth and intestine morphology of T. ovatus, enhance the plasma immune indexes and hepatic antioxidative capacity, inhibit the expression of intestinal Nf-κb and proinflammatory cytokine mRNA, and improve the expression of antiinflammatory cytokine mRNA. The suitable level of AO in diet is 0.7 g·kg⁻¹.