Abstract: The juveniles of tilapia (Oreochromis niloticus×O.Areus) were exposed to ammonia-N (0, 2.5 mg · L^-1, 5 mg · L^-1, 10 mg · L^-1 and 20 mg · L^-1) for 0, 12 h, 24 h, 48 h and 72 h to evaluate the effect of ammonia-N stress on their non-specific immunity. Results show that the activity of serum lysozyme decreased significantly with extension of stress time (P < 0.05). The total antioxidant capacity (T-AOC) and activity of antioxidase in liver were significantly affected. The activity of T-AOC and total superoxide dismutase (T-SOD) of fish exposed to ammonia-N were initially decreasing then increasing(P < 0.05). Activities of catalase (CAT) and glutathione peroxidase (GSH-Px) were correlated with concentrations of ammonia-N. Fish exposed to lower concentrations (2.5 mg · L^-1, 5 mg · L^-1) showed decreasing CAT activity within 24 h (P < 0.05), while those exposed to higher concentrations (10 mg · L^-1, 20 mg · L^-1) showed initially increasing then decreasing activity of CAT. Except for the highest concentration groups, fish exposed to ammonia-N showed induction activity of GSH-Px (P < 0.05). Under the experimental conditions, non-specific immunity of tilapia was affected by ammonia-N stress, and the impact was increasing with increasing concentration and extension of time.