The objective of this experiment was to examine the effects of heat stress (33 °C) or heat stress with ascorbic acid (AA) supplementation [3g AA/kg feed on dry matter (DM) basis on behavior, growth performance, mortality rate and some blood constituents in Oreochromis niloticus. A total number of 120 fish were alloted according to body weight and length into three groups. The first group (A) served as control. The second group (B) exposed to high water temperature (33 °C), while the third group (C) exposed to high water temperature (33 °C) and supplemented with dietary ascorbic acid, 3g/kg DM. Daily changes in their behavior, feeding pattern and mortality rate as well as weekly body weight and body length were recorded on individual basis. Blood samples were taken biweekly. At the end of the experiment all fish were slaughtered and the liver, spleen and gonads were weighed. The experimental period lasted for six weeks. Fish which were exposed to high water temperature exhibited abnormal behavior and higher mortality rate as well as lower body weight, body length and relative body weights, i.e. HSI, SSI, FGSI and mGSI. Also, heat stress increased (P<0.01) hemoglobin and hematocrit values. Serum glucose concentration in group B was twice as much as in group A. Serum total protein concentration was lower in group B than in group A due to the decrease in serum globulin rather than albumin. Heat stress increased (P<0.05) the overall means of serum AST and ALT concentrations by about 58 and 28%, respectively, while it decreased the concentration of serum alkaline phosphatase. Such negative effects of heat stress on O. niloticus raised in closed system could be counteracted by dietary ascorbic acid at 3g/kg DM, since its supplementation maintained normal behavior, body growth, mortality rate and body relative weights in fish exposed to heat stress. Also, ascorbic acid is most beneficial in counteracting the adverse effects of high water temperature on serum metabolites and enzymes. It was concluded that addition of dietary ascorbic acid (3g/kg DM) may be useful to counteract the adverse effects of high water temperature on Oreochromis niloticus in intensive closed system. Key Words: Fish, ascorbic acid, heat stress. blood