The usage of doxorubicin for a long time and large doses cause side effects of cardiotoxicity, hepatotoxic of bone marrow suppression, toxicity of hematology and resistance. Therefore, it needs a co-chemotherapy agent which is potential to be the companion agents of chemotherapy on the step of breast cancer treatment. Atenolol is a member of β-blockers (B1) selective group. It may reduce the rate of breast cancer cells progress which β1 and β2 adrenergic receptors play the roles in proliferation, migration, adhesion and metastasis effects.This study aims to determine the potential of atenolol to increase the sensitivity of breast cancer cells and reduce the side effects of doxorubicin by knowing the scores of IC50 (Inhibition Concentration), CI (Combination Index), and its ability to inhibit the proliferation of T47D breast cancer cells. A single dose of atenolol test showed the moderate cytotoxicity potential with IC50 285.51 μg / mL. The combination of atenolol with doxorubicin and various concentrations cause a mild antagonistic effect at the low dose and synergistically strong atenolol concentration. The combination of atenolol with half of DOX dose can inhibit increasing anti-proliferation effects better since it reduces the doses of DOX. So, it is possible to reduce the cytotoxic effects toward the normal cells.