Abstract: The juveniles of tilapia (Oreochromis niloticusO.areus) were exposed to ammonia-N (0, 2.5 mgL-1, 5 mgL-1, 10 mgL-1 and 20 mgL-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 (P0.05). 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 decreasingthen increasing(P0.05). Activities of catalase (CAT) and glutathione peroxidase (GSH-Px) were correlated with concentrations of ammonia-N. Fish exposed to lower concentrations (2.5 mgL-1, 5 mgL-1) showed decreasingCATactivity within 24 h (P0.05), whilethose exposed to higher concentrations (10 mgL-1, 20 mgL-1) showed initially increasingthen decreasing activity of CAT. Except for the highest concentration groups, fish exposed to ammonia-N showed induction activity of GSH-Px (P0.05). Under the experimental conditions, non-specific immunity of tilapia was affected by ammonia-N stress, and the impact was increasing withincreasing concentration and extension of time.