Abstract: To evaluate the effects of dietary chitosan oligosaccharide (COS) on growth performance, feed utilization, specific and non-specific immunity of hybrid yellow catfish Tachysurus fulvidraco♀× Tachysurus vachelli♂, a total of 420 fish with the initial body weight 33—40 g were randomly assigned to 4 treatments with 3 replicates. The fish were fed experimental diets supplemented with 0, 40, 80 and 120 mg/kg COS for 8 weeks. Specific growth rate (SGR) in 120 mg/kg COS group was significantly higher than that of 0 and 40 mg/kg groups (P<0.05). Feed conversion rate (FCR) of the 0 COS group was significantly higher than that of 120 mg/kg group. As the increase of COS dose, the variation trend of protein efficiency ratio (PER) was opposite to that of FCR. White blood cell count (WBC) was not significantly affected by COS treatment (P>0.05). COS significantly increased leukocyte Phagocytic index (PI) (P<0.05). Lymphocyte transformation rate (LTR) increased significantly with the increased COS dose from 0 to 80 mg/kg (P<0.05), and then remained unchanged after 80 mg/kg dosage (P>0.05). Lysozyme activity and IgM levels in 80 and 120 mg/kg COS treatment groups were significantly higher than the other two groups (P<0.05). The present results suggest that, under the current conditions, 80—120 mg/kg supplement of COS (effective dose) can increase the SGR, reduce the FCR, promote leukocyte phagocytosis and lymphocyte turnover, and increase lysozyme activity and immunoglobulin levels.