Aims Breast cancer is the most common cancer among women that usually begins with abnormal growth and division of different breast cells. There is some evidence that there is an inverse relationship between levels of antioxidants and the risk of breast cancer. The purpose of this study was to investigate the effect of 8 weeks resistance training on some antioxidant/oxidative indexes in postmenopausal women with breast cancer. Materials & Methods In this semi-experimental study, in 2014, 30 postmenopausal women with breast cancer in Khorramabad city were selected by available sampling method and randomly divided into 2 groups: experimental group (n=16) and control group (n=14). The experimental group performed resistance training for 8 weeks, which started from 30% 1RM (one repetition maximum) and reached 45-50% 1RM at the end of the eighth week. Blood samples were taken from subjects for the measurement of superoxide dismutase (SOD), glutathione peroxidase (GPX) and malondialdehyde (MDA) serum levels, 48 hours before and after the training protocol. Data were analyzed by SPSS 19 software using independent t-test and dependent t-test. Findings After performing 8 weeks of resistance training, serum levels of SOD and GPX significantly increased and MDA levels decreased significantly (p<0.05), but changes in control group were not significant (p>0.05). Conclusion Resistance training for 8 weeks increases antioxidant indexes and decreases oxidative indexes in postmenopausal women with breast cancer.