Breast cancer is the most common cancer and cause of cancer death among women around the world. The inhibitory effect of calcium in cell proliferation and incidence of breast cancer has been identified in some studies, although the exact of its mechanism is not clear very well. To further study the potential inhibitory effects of calcium in breast cancer, we used computational methods, the Becke three-parameter Lee-Yang-Parr (B3LYP). The results indicated that Calcium (Ca2+), estrone (E1) and Calcium – estrone complex, all after optimization, had the positive frequency and negative energies. The amount of the Gibbs free energy change (ΔGa) for Calcium absorption reaction on estrone was negative that represents spontaneous calcium absorption process on estrone. Therefore, it is possible that calcium reduce the cell proliferation and carcinogenesis by estrogen through producing a stable complex with estrone, changing estrone conformation and suppressing conversion of estrone to estradiol. Key Words: