Breast cancer is the leading cause of cancer-related deaths in women worldwide. Tanshinone IIA (Tan-IIA) is one of the pure compounds from Salviae miltiorrhizae radix (Danshen). Tan-IIA can inhibit human breast cancer cells but the molecular mechanisms are not well understood. Our previous study showed that Tan-IIA can inhibit hep-J5 human hepatocellular carcinoma cells through the endoplasmic reticulum (ER) stress-induced apoptotic pathway. In the present study, we evaluated the effects of Tan-IIA on BT-20 human breast cancer cells and assessed the involvement of the ER-stress-apoptotic pathway. The cytotoxicity of Tan-IIA in BT-20 cells was measured by the MTT assay. The cell cycles were analyzed by flow cytometry. The expression of ER stress-related proteins in BT-20 cells treated with Tan-IIA were evaluated by western blotting and immunocytochemical staining. These results showed that Tan-IIA can inhibit BT-20 cells and increase the sub-G1 phase in a time- and dose-dependent manner. Tan-IIA could increase the protein expression of caspase 12, GADD153, caspase 3, phospho-JNK, phospho-p38 and Bax, but decreased Bcl-xl and phospho-ERK expression in BT-20 cells. These findings indicate that Tan-IIA possesses therapeutic potential for human breast cancer BT-20 cells; one of the molecular mechanisms may be through inducing ER stress and the MAPK pathway to induce apoptosis and inhibit proliferation.