The ability of tanshinone IIA (Tan-IIA) to inhibit the proliferation of human breast cancer cell lines in vitro and in vivo is well documented. However, the molecular mechanisms have not been fully elucidated. In the present study, MDA-MB-231 cells were treated with different concentrations of Tan-IIA for 48 h, followed by protein extraction for western blotting. For an in vivo study, MDA-MB-231 cells were implanted directly into female SCID mice which were divided randomly into three groups to be treated with vehicle, Tan-IIA (20 mg/kg) and Tan-IIA (60 mg/kg) every other day orally, with treatment starting 4 weeks after inoculation with the MDA-MB-231 cells. The results showed that Tan-IIA inhibited the proliferation of MDA-MB-231 cells and decreased the protein expression of LC3-II and Erb-B2 in vitro. Treatment with Tan-IIA (20 or 60 mg/kg) for 90 days resulted in a reduction in tumor size and weight compared to the control group. The protein expression of NF-κBp65 was reduced, while caspase-3 was up-regulated compared to the control group. These findings indicate that Tan-IIA inhibits tumor growth in a MDA-MB-231 xenograft animal model. One of the molecular mechanisms may be through a decrease in NF-κBp65 and an increase in caspase-3 expression.