Breast cancer is the second most prevalent cancer among women and its incidence is amplifying alarmingly. Since genetic factors are believed to account for only 10% of the reported cases, remaining the environmental factors, including diet are thought to play a significant role in predisposing breast cancer. Many bioactive compounds have been reported for their anticancer potential. One among the bioactive compound 3, 3′-Diindolylmethane (DIM) is a phytochemical possess a wide array of pharmacological activities such as anti-proliferative and anti-oxidant properties. Its properties such as poor water solubility and low bioavailability have hampered its clinical development. Therefore, it is a great interest to study whether the nano formulation for DIM with chitosan for its enhanced potential, the present study was aimed to evaluate the chemotherapeutic potential of 3, 3′- Diindolylmethane encapsulated chitosan nanoparticles (DIM@CS-NP) on 7,12-Dimethylbenz(a)anthracene (DMBA) induced mammary carcinoma in rats. DMBA was induced in a single subcutaneous injection of 25 mg/ kg body weight to each rat. In the present study, we investigated the altered activities of lipid peroxidation, enzymatic antioxidants (SOD, CAT, GPx) and non- enzymatic antioxidant (GSH) in plasma, liver and mammary tissue, supported by histopathological study of mammary tissues. We evaluated the changes in the body weight of control and experimental animals. There was a significant decrease in the final body weight of tumor bearing animals, when compared to control animals. Also, we observed a diminished cellular antioxidant status and the elevated oxidant levels in plasma, liver, mammary tissues of DMBA induced rats. However, administration of DIM@CS-NP significantly increased the mean final body weight when compared with DMBA induced animals. Oral supplementation of different doses of DIM@CS-NP (0.5, 1, 2 mg/kg BW), significantly renovated the activities of cellular antioxidants and ultimately diminished the levels of lipid peroxidation, which pointed towards suppression of preneoplastic lesions, thereby reduced the cancer risk, and significant improvement in the levels of enzymatic (SOD, CAT, GPx) and non- enzymatic antioxidant (GSH) in the plasma, liver and mammary tissue. Based on the above finding we conclude the nano formulation of DIM provides a novel therapeutic regime for mammary cancer.