The effect of copper concentrations on alterations in hematological and biochemical parameters as well as  reproductive hormones of the Nile tilapia Oreochromis niloticus was investigated. The copper sulfate at sub-chronic concentrations 25, 50 and 75 µg/L on Oreochromis niloticus for 90 days of exposure. Significant changes (P ˂ 0.05) in almost all hematological aspects were found in the all groups exposed to different concentrations of copper 25, 50 and 75 µg/L for 90 days. Biochemical analysis revealed various significant (P ˂ 0.05) differences among the all groups that exposed to 25, 50 and 75 µg/L of copper for 90 days of exposure. There was a significant difference in the copper concentrations in the tissues (P ˂ 0.05) among all treated groups; in liver, gills and kidney. Glucose and cortisol in fish plasma increased significantly, while total protein and total lipids decreased significantly due to copper stress. The obtained results showed also that copper stress was harmful to the fish liver and kidney, where plasma aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, uric acid, and creatinine values were significantly increased with the increasing exposure of copper on Oreochromis niloticus for 90 days compared to the control one. Plasma follicular stimulating hormones (FSH), 17ß estradiol (E2) and testosterone (T) were decreased significantly in fish with the increasing exposure of copper concentrations. Among antioxidative enzymes, significant changes were revealed mainly in plasma ceruplasmin and glutathione reductase and glutathione peroxidase activity in liver and gills (P ˂ 0.05. The results demonstrate the deleterious influence of copper on Oreochromis niloticus, even at low doses. So, it is recommended to not use copper too much during  treatment  of the water that used in growing the fish.