Aerobic training reduces breast cancer tumor growth. This study evaluated the expression of Bcl-2, miR-15, and Bcl-2 protein as a positive mechanism induced by aerobic training. Twenty BALB/c mice (5–6 weeks, 16–17 g) were injected with MC4-L2 mammary cancer cells. Then, they were randomly divided to exercise and control groups (n=10) and 48 hours after the last exercise session, they were scarified and blood and tissue samples were collected and stored in -70oC. Expression of miR-15 and Bcl-2 were analyzed with Real- time PCR and ELISA. The expression level of Bcl-2 gene and protein as well as and tumor growth were significantly decreased in exercise group compared with control group (P<0.001). Also, miR-15 expression increased significantly in exercise group compared with control group (P<0.001). Overall, it seems that overexpression of miR-15 and decreased expression of Bcl-2 gene and protein induced by aerobic exercise training was effective in reducing tumor growth rate.a