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Background & Objective: Free radicals are formed in all living organisms during normal cell metabolism. Patients with chronic renal failure, which regularly dialyzed are the candidates for, free radical damages. The aim of this study with the discriminative information was to evaluate the effect of hemodialysis on Lipid peroxidation (The level of Lipid peroxidation expressed as malondialdehyde) and erythrocyte antioxidant enzyme (Glutathione peroxidase) before and after the dialysis and compared with control group, to find out the effect of hemodialysis on the level of Lipid peroxidation of plasma and the activity of erythrocyte antioxidant enzyme. Materials & Methods: This investigation was an analytical type of study and sampling procedure was according to purposive method. 22 patients with chronic renal failure (CRF) disease who were hemodialysed at 5th Azar Hospital of Gorgan dialysis center and 22 age and sex matched healthy control were recruited for this study. The data was analyzed by SPSS software using T-test. Results: Plasma malondialdehyde showed significant difference between the predialysis (And control group. It was increased in the postdialysis group (2.32±0.38 nmol/ml) when compared with predialysis (1.27±0.23 nmol/ml) and control group (0.98±0.17 nmol/ml). Erythrocyte antioxidant enzyme was decreased in postdialysis group (22.26±4.76 unit/gram hemoglobin) when compared with predialysis (29.66±5.95 unit/gram hemoglobin) and control group (37.52±6.26 unit/gram hemoglobin). There was also significant difference between control and predialysis group. Erythrocyte antioxidant enzyme was lower than control group in dialysis group. Conclusion: The observation of meaningful differences in reduction of erythrocyte antioxidant enzyme and increasing level of plasma Lipid peroxidation in the hemodialysed patients after the process of dialysis, maybe related with the patient, uremia, dialysis membrane (The loosing antioxidant enzyme through this membrane), and the dialysis process (May increase Lipid peroxidation during the dialysis process). These states of affairs may play an important role in progress of cardiovascular abnormality in hemodialysed patients. Due to this conditions a review of hemodialysis membrane, the techniques used in the dialysis, the consumption of various oral antioxidant, the elimination of active oxygens from the dialysis surrounding are among the measures which can prevent sudden cardiovascular abnormality in the hemodialysis patients and ultimately these important factors up-grade the patients quality of life.