To investigate effects of chitosan on nonspecific immunity in grass carps, five test groups were used, which consisted of control group, antibiotics group, 0.25%, 0.50% and 1.00% chitosan addition group. 450 grass carps with average body weight (70.61±1.29) g were randomly allocated to five groups with three replicates and with 30 grass carps in each replicate and were fed for 40 d. The results showed that, post kidney body index increased significantly by adding chitosan, while head kidney body index and spleen body index showed no significant difference (P>0.05) among groups. 0.5% chitosan significantly impacted on serum total protein in grass carp (P<0.05), however serum SOD and hepatopanereas SOD showed no significant difference (P>0.05) among groups. Above results suggested that, chitosan could significantly increase nonspecific immunity in grass carps and the appropriate chitosan addition was 0.50%.