Breast cancer is the complex and heterogenous malignancy caused as a result of interaction of hormonal, environmental and genetic factors. Major advances made in the field of breast cancer research have emanated from studies involving human breast cancer cell lines MCF-7 and T47D. Insulin act as a potent mitogen in normal mammary tissue and breast cancer cells in culture. Insulin receptor is overexpressed in several human breast cancers and this overexpression results in transformed phenotype in human mammary epithelial cells. Experimental study has shown that Insulin induces significant mitogenic effect in MCF-7 and T47D cells in dose-dependent manner. 100nM concentration of Insulin enhances the growth of MCF-7 and T47D cells by ? 7.3 and ? 5 folds respectively in comparison to control. The antidiabetic drug metformin lowers risk of breast cancer via direct action (Insulin independent) and indirect action (Insulin dependent). The treatment of the hyperinsulinemia with the metformin will lower the circulating levels of insulin and improve insulin sensitivity, thus results into potential decrease in the incidence of breast cancer. The results show that MCF-7 and T47D cells growth inhibited over control at high doses (10 mM concentration) of metformin. MCF-7 and T47D cells also show decrease in insulin induced cell proliferation at higher doses of metformin.