Background: Exercise has positive effects on breast cancer evolution, including prevention, medical treatment, and aftercare clinical settings. Plus, elevated serum levels of CEA and CA15-3 as prognostic indicators were identified in patients with breast cancer. So, this study aimed to investigate the changes of CEA and CA15-3 biomarkers in the breast cancer patients following eight weeks of aerobic exercise. Methods: In this quasi-experimental study, 15 females with breast cancer with mean age (44.46±17.15 years), weight (70.53±5.18 kg) and body mass index (27.58±2.18 kg/m2) were randomly selected and performed aerobic training (for 8 weeks,3 sessions per week, with an intensity of 30%-60% heart rate reserve). Before and after the intervention, while all patients were fasting for 12 hours, the levels of CEA and CA15-3 were measured. The data obtained were analyzed, using paired sample t-test at a significance level of P<0.05. Results: Findings showed that the tumor marker (CA15-3) decreased following 8 weeks of aerobic exercise although it was not significant (P=0.091); CEA level did not change significantly after exercise, either (P=0.542), but a significant decrease was observed in body mass index, body fat percentage and weight values after exercise (p=0.001) . Conclusion: Elevated CA15-3 and CEA which are considered as the diagnosis of metastatic breast cancer was not observed in the study. Exercise could reduce CA15-3 insignificantly. Also, aerobic exercise for eight weeks improved body composition indices, without increasing the level of fatigue or stress values, which may contribute to the prevention of the cancer prevalence in middle-aged women

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