A feeding experiment was conducted to examine the effects of dietary chitosan on growth, immunity and disease resistance for Nile tilapia, Oreoehromis niloticus with initial weight of (99.98±2.79) g. Nile tilapia (Oreoehromis niloticus) were inoculated with Aeromonas hydrophila and were fed with diets supplemented with appropriate amount of chitosan. 30 days after the inoculation, the agglutinating antibody titer were examined; 60 days after the inoculation, the relative rate of weight gain, specific growth rate, feed efficiency, protein efficiency, acid phosphatase, lysozyme activity and superoxide dismutase activity and bacterial relative percent survival by challenging with live A. hydrophila. The results showed that adding chitosan with a ratio of 0.6% into the diets can promote their growth and enhance the immune response, and chitosan can improve their non-specific defence and the resistance of Nile tilapia to immune response A. hydrophila.