Abstract: ObjectiveThe effects of different concentrations of ammonia in water on growth and physiological and biochemical mechanism of Oreochromis niloticus were studied to provide theoretical references for healthy aquaculture of tilapia.MethodWith juvenile O. niloticus as the research object, one control group (A0) and four experimental groups were set at the concentration of 3.49(A1), 6.99(A2), 13.97(A3) and 27.94(A4) mg/L, respectively. 30 days after feed-ing, growth, blood routine parameters and enzymatic activity of fish were mensurated under ammonia stress. ResultUnder chronic ammonia stress, surface mucus of O. niloticus increased at a slower swimming speed. The fin rays were defected and the pectoral fin base congested. Some dead individuals had tail fester, gill hyperemia, little food in intes-tine and white liver, which were especially obvious in the A4 treatment group (27.94 mg/L) .The results showed that O. niloticus weight gain rate (WGR) and specific growth rate (SGR) decreased with the increasing of ammonia nitrogen con-centration. The number of white cells in each group was significantly higher than that of the control group (P<0.05, the same hereinafter). Hemoglobin content (HGB) was significantly lower (decrease by 8.98%-15.50%). Hematokrit also fell (decrease by 5.80%-12.06%). Acid phosphatase (ACP) and alkaline phosphatase (AKP) activity in tilapia liver showed a rising trend , superoxide dismutase ( SOD ) and glutamic-pyruvic transaminase ( GPT ) were significantly lower than those of the control. ConclusionChronic ammonia stress has great influence on growth, blood routine pa-rameters and liver enzyme activity of O. niloticus. Immunity , anti-stress ability and disease resistant ability of fish de-crease under chronic ammonia stress. In practice, close attention should be paid to ammonia changes in aquaculture production in order to reduce its toxic and side effect on O. niloticus.