Abstract: To investigate the effects of chronic external ammonia stress on growth, immunity and metabolism of juvenile GIFT tilapia (Oreochromis niloticus), we designed five concentrations of ammonia, including 0 mgL-1(A0), 1.63 mgL-1(A1), 3.25 mgL-1(A2), 6.51 mgL-1(A3) and 13.01 mgL-1(A4). The results show that after 30-day feeding, the juveniles weight gain rate (WGR) and specific growth rate (SGR) decreased significantly with increasing concentration of ammonia. For immunity, the number of white blood cells (WBC) increased but that of hemoglobin (HGB) decreased with increasing concentration of ammonia. Serum albumin(ALB) concentration and superoxide dismutase(SOD) activities of A3 and A4 were significantly less than A0, while alkaline phosphatase (AKP) concentration of A4 was significantly less than A0. For metabolization, serum triglycerides(TG) concentration of A3 and A4 were significantly less than A0, while total cholesterol (TC) concentration of A1~A4 were significantly less than A0. Serum alanine transaminase(ALT) activities of A3 and A4 were significantly less than A1 and A2. Meanwhile, only aspartate transaminase (AST) activities of A4 was less than A0. It is revealed that the growth, nonspecific immunity and metabolism of juvenile GIFT tilapia would be inhibited under chronic ammonia stress; change of ammonia concentration in tilapia aquaculture should be paid more attention to.