This experiment was conducted at the Central Laboratory for aquaculture research, Ministry of Agriculture, Egypt. This study was devoted to study the effect of different sub lethal concentrations of total ammonia on growth performance and some physiological parameters of Nile tilapia (Oreochromis niloticus). One hundred and forty four Nile tilapia fish with average weight 52.3±0.81 g, were reared in twelve aquaria of 60x40x50 cm represented four replicated groups, the first group used as a control group (fresh water with no adding of ammonia) and the other three groups were treated with different concentrations of ammonia determined as 3, 5 and 7 mg/l. The experimental period was two months. Dissolved oxygen, temperature, salinity, pH, total alkalinity and total hardness were constantly measured twice weekly. Under these treatments, fish growth performance, Hemoglobin concentration (Hb), erythrocyte counts (RBC), hematocrite (Htc) percentage, other blood indices, total iron, total iron binding capacity (TIBC), insulin, amylase and lipase were measured. The results showed that there was significant decrease in growth performance, blood indices, serum total iron, total iron binding capacity (TIBC), amylase and lipase by increasing ammonia concentration. It could be concluded that increasing concentrations of total ammonia in water ponds more than permissible limits decreases the levels of digestive enzymes and TIBC which reflecting on reducing growth performance of Nile tilapia.