This experiment was done to evaluate the effects of ammonium nitrate on Nile tilapia. The fish were exposed to sub lethal concentrations of ammonium 2, 4 and 8 mg L-1 of ammonium nitrate respectively for a period of 7 days, 14 days and 21 days respectively to study the changes in hematological parameters and the serum constitutes. General decline in the hematological parameters and the serum constitutes were observed beside the rate of serum constitutes decreased progressively with the increase of toxicant concentration and duration of the exposure. It can be concluded from this study that disturbance in the RBCs and WBCs, hematocrit value, hemoglobin content, serum total protein, serum glucose level, serum activities of (AST, ALT) and (creatinine and uric acid) concentrations as a result of stress of ammonium nitrate on Nile tilapia O. niloticus reflect the disturbance in all metabolic function and can be used as marker of pollution. Also, the changes in the hematological parameters indicate that they can be used as indicators of ammonium nitrate related stress in fish on exposure to elevated ammonium nitrate levels in the water.