Red blood cell (RBC) units undergo metabolic, structural, and biochemical changes known as "storage lesions" that can reduce the survival and quality of RBCs. The use of antioxidants such as α-tocopherol may help to improve the quality of RBC units by reducing oxidative stress. The aim of this study was to determine the antioxidant effect of α-tocopherol in RBC units containing citrate-phosphate-dextrose solution with adenine (CPDA1) stored at 1°C-6°C for 35 days. Four RBC units containing CPDA1 were divided into four equal satellite bags. Three bags were supplemented with 0.125, 0.625, and 3.125 mM concentrations of α-tocopherol as test groups. One bag was supplemented with ethanol (0.5%) as a control group. They were stored at 1°C-6°C for 35 days. Malondialdehyde (MDA) concentration, total antioxidant capacity (TAC), and hemolysis index (HI) were measured on days 0, 7, 14, 21, 28, and 35. In all groups, MDA concentration and HI increased and TAC decreased (P < 0.05). MDA concentration and HI in the 3.125 mM of the α-tocopherol group had a lower increase compared to the other test and control groups. Supplementation of RBC units with α-tocopherol resulted in a significant increase of TAC in all three groups compared to the control group (P < 0.05) and had a lower reduction during storage. Supplementation of RBC units with α-tocopherol improves the quality of RBC units by decreasing lipid peroxidation and hemolysis and by increasing TAC. Among the mentioned concentrations, 3.125 mM of α-tocopherol had a significantly more antioxidant effect.