Introduction: Breast cancer is a malignant tumor characterized by uncontrolled proliferation of cancerous cells by exposure to the risk of death. Therefore, this study aimed to investigate the effect of eight weeks of aerobic exercise on cytochrome C, caspase 9 and tumor volume in mice with breast cancer. Methods: In this experimental study, 20 female balb/c mice (age: five to six weeks) were injected with MC4-L2 cancerous cells. They were then randomly assigned to two groups of 10 mice including experimental and control groups. Experimental group performed exercise for eight weeks (55-70% of the aerobic capacity of the mice). 48 hours after the last training session, the mice were sacrificed and their tumors tissue samples were removed and stored at -70 ° C and the level of factor expression of cytochrome C and caspase-9 of tumor tissue was measured by Real Time-PCR. Data were analyzed by SPSS21 using independent sample t- test (p≤ 0.05). Results: There was a significant difference between the two groups of control and aerobic exercise in the cytochrome C (P = 0.001) and caspase 9 (P = 0.039), and the levels were higher in the exercise group than in the control group. Also, regarding tumor volume index, tumor volume in the exercise group was significantly less than that in the control group (P = 0.001). Conclusion: In general, the results of this study confirm the positive role of eight weeks of aerobic exercise by stimulating upstream factors affecting the process of cancerous cell apoptosis. تفاصيل المقالة