This study was conducted to investigate the effect of vitamin E treatment on reproductive performance of cows synchronized with different synchronization methods. Eigthy-four holstein cows were used as material. The cows that were synchronized by double PGF_2α injections 11 days apart, ovsynch and CIDR insertion were divided into two groups as control and treatment (vitamin E) within each synchronization group. Ten ml blood samples were taken from all groups, and then 4 ml physiological saline was injected to all control groups, 4 ml vitamin E (300 mg/ 2 ml) injected to all treatment groups. Vitamin E and MDA levels were determined in all blood samples. All the animals were examined for pregnancy on day 60 after insemination. No statistically significant difference was determined between different synchronization groups, and between control and treatment subgroups within same synchronization group with respect to pregnancy rates. There was a statistically significant difference between MDA levels of first and second blood samples in both control (p<0.01) and treatment (p<0.001) subgroup cows which were synchronized with CIDR synchronization method. In conclusion, although administration of vitamin E to cows which were synchronized with different synchronization methods provides a slight increase in pregnancy rates, it significantly prevents the CIDR induced-lipid peroxidation.