The current study was aimed to explore the effects of chitosan oligosaccharide (COS) on the growth performance, blood biochemical indexes and immune function of weaned lambs. A total of forty healthy weaned lambs of Dorset F2 generation with similar body weight were selected and randomly divided into four groups. The basic diet was supplemented with 50 mg/kg, 100 mg/kgand 150 mg/kgCOS, and a blank control group was also set, with the test periodof 42 days.The results showed that the final body weight and ADG of Groups Ⅱ andⅢ were significantly higher than those in the control group. In contrast, the FCRs of Groups Ⅱ andⅢ were significantly lower than those in the control group. The alkaline phosphatase activity and cholesterol content of Groups Ⅱ and Ⅲ were significantly lower than those in the control group. The immunoglobulin A content of Groups Ⅱ andⅢ was significantly higher than that in the control group, and the immunoglobulin G content of Groups Ⅱ andⅢ was significantly higher than that in the control group. The content of tumor necrosis factor-alpha in Groups Ⅱ and Ⅲ was significantly higher than that in the control group. The addition of COS to the diet of weaned lambs can improve their growth performance and blood biochemical indexes and enhance the body'simmunity, and the optimum supplement of dietary COS level should be 100 mg/kg.