Introduction: The decorin is a myokine that modulates the growth and the development of the cancer cells through the interaction with TGF-β. The present study is aimed to evaluate the protective effects of 8 weeks interval aerobic exercise on the expression of decorin and TGF-β, and tumor volume in a typical animal of breast tumor. Materials and Methods: In a exprimental research, forty adult female Balb/c mices were randomly divided into four groups: control, tumor, exercise, and exercise with the tumor. The main exercise was performed four weeks before and after tumor with 50-70 percent of the maximum power of mices. The mices were cancerous by subcutaneous surgery with tumor of adenocarcinoma of mice breast in the tumor group and the exercise group with the tumor. Tumor growth was measured weekly. At the end of the study, tumor and soleus muscle were extracted by surgery. Real-Time PCR method was used to evaluate the expression of decorin and TGF-β and data were analyzed by t-test, one-way ANOVA and post-hoc methods with considering the significant level of p<0.05. Results: The tumor growth rate in tumor group was significantly higher than the exercise group with the tumor. A significant difference confirmed between four groups In the level of decorin expression in soleus muscle (F=12.30 , P=0.0023). The Post hoc test showed that 8 weeks of interval aerobic training significantly increased the expression of decorum gene in soleus muscle in two pairs of exercise group compared with control group and exercise group with tumor compared to tumor group (P=0.02). The expression of TGF-β in tumor tissue showed a significant reduction in exercise group with tumor compared with tumor group. Conclusion: Our results showed that interval aerobic training probably contributes to decreasing the growth of breast cancer cells by increasing the expression of decorin and decreasing the expression of TGF-β.