Abstract: To study the effects of galacto-oligosaccharides(GOS) on the growth performance, serum immunological and biochemical factors of cobia (Rachycentron canadum), we supplemented GOS in the feed at different levels (0, 0.2%, 0.4%, 0.8% and 1.6%) and fed the cobia for 8 weeks. The addition level lower than 0.4% improves the growth rate insignificantly (P>0.05).The activity of serum lysosome and superoxide dismutase is improved significantly by GOS supplementation. However, the catalysase activity is inhibited significantly. The serum biochemical factors including the total protein, albumin, globulin, total bilirubin and direct bilirubin are influenced significantly by GOS supplementation, but no significant influence is observed on the indirect bilirubin. The concentration of serum cholesterol is significantly higher in the experimental group supplemented with 0.2% GOS than in the control, and the concentration of aspartate aminotransferase (AST) is significantly lower in the group supplemented with 0.4% GOS than in the control. The AKP activity decreases with the increase of GOS concentration when GOS concentration is lower than 0.8%. In conclusion, the growth rate and immunological function of cobia can be improved by GOS supplementation in the feed at proper level, and the optimal concentration is 0.2%~0.4%.