Introduction: Oxidative stress occurs as the result of imbalance between free radicals production and anti-oxidant power of tissues. In the recent studies it is suggested that hemodialysis increases formation of oxygen radicals resulting in reduced anti-oxidant capacity of different tissues. The aim of the present study was to investigate the changes in red blood cells superoxide dismutase activity (SOD, an anti-oxidant enzyme) in hemodialysis patients.Methods: Blood samples were taken from 17 hemodialysis patients and 12 healthy people (control group). Packed cells were extracted from blood samples by centrifuging. SOD activities were assayed in packed blood cells in both groups by Paoletti’s method.Results: SOD activity was significantly lower in hemodialysis patients compared with control group (P<0.05).Conclusion: The result of the present study suggests that anti-oxidant capacity of blood in hemodialysis patients may decreases due to increased oxidative stress.