This experiment was conducted to study the effects of xylo-oligosaccharides on non-specific immune function of grass carp (Ctenopharyngodon idellus). Five hundred and twenty-five healthy grass carp with basically the same size and body weight were randomly assigned to 5 groups with 3 replicates in each group and 35 fish in each replicate. Grass carp in 5 groups were fed with the basal diets supplemented with 0 (group A0), 1 (group A1), 2 (group A2), 4 (group A3) and 6 g/kg (group A4) xylo-oligosaccharides, respectively. The activities of blood leukocyte phagocytic, serum lysozyme, and serum superoxide dismutase, and the content of serum complement C3 were determined on day 0, 7, 14, 28 and 56, respectively. Calculating the mortality after 14 d of challenge test when the culture experiment (lasted for 56 d) was finished. The results showed that the immune indices of experimental groups were enhanced compared with the control group in the whole experiment. Among them, the leukocyte phagocytic activity, serum lysozyme activity, serum superoxide dismutase activity and serum complement C3 content in group A3 were significantly higher than those in the control group (P<0.05), which reached to the top level on day 28, 28, 7, 28 of the experiment, respectively. The group A3 also had the lowest mortality rate among the 5 groups. These results indicate that xylo-oligosaccharides can enhance the non-specific immune function of grass carp, and the optimum addition level is 4 g/kg.［Chinese Journal of Animal Nutrition, 2010，22（6）:1687-1693］