In order to study the effect of fructooligosaccharides on non-specific immunity of Ctenopharyngodon idellus,the fish were fed with diets containing fructooligosaccharides 0,0.5,2 and 4 g/kg respectively for 56 days.Phagocytic activity of leukocytes,lysozyme activity and the content of complement C3 and C4 in serum were studied on 0,7th,14th,21st,28th,42nd and 56th day.The results showed that the immune indexes of experiment groups were markedly enhanced,especially in the group of 2 g/kg fructooligosaccharides in diet (P<0.05).The group with 2 g/kg addition in diet had lower mortality than the other groups.We suggested that 2 g/kg addition was suitable from the aspects of immunity and economy.