Background: Tanshinone IIA (Tan IIA), a lipophilic constituent from Salvia miltiorrhiza Bunge, has shown a promising cardioprotective effect including anti-atherosclerosis. This study aims at exploring Tan IIA's anti-inflammatory and immune-regulating roles in stabilizing vulnerable atherosclerotic plaque in ApoE-deficient (ApoE(-/-)) mice. Methods: Male ApoE(-/-) mice (6 weeks) were fed with a high-fat diet for 13 weeks and then randomized to the model group (MOD) or Tan IIA groups [high dose: 90 mg/kg/day (HT), moderate dose: 30 mg/kg/day (MT), low dose: 10 mg/kg/day (LT)] or the atorvastatin group (5 mg/kg/day, ATO) for 13 weeks. Male C57BL/6 mice (6 weeks) were fed with ordinary rodent chow as control. The plaque stability was evaluated according to the morphology and composition of aortic atherosclerotic (AS) plaque in H&E staining and Movat staining sections by calculating the area of extracellular lipid, collagenous fiber, and foam cells to the plaque. The expression of the Toll-like receptor 4 (TLR4)/myeloid differentiation factor88 (MyD88)/nuclear factor-kappa B (NF-kappa B) signal pathway in aorta fractions was determined by immunohistochemistry. Serum levels of blood lipid were measured by turbidimetric inhibition immunoassay. The concentrations of tumor necrosis factor-alpha (TNF-alpha) and monocyte chemoattractant protein-1 (MCP-1) were detected by cytometric bead array. Results: Tan IIA stabilized aortic plaque with a striking reduction in the area of extracellular lipid (ATO: 13.15 +/- 1.2%, HT: 12.2 +/- 1.64%, MT: 13.93 +/- 1.59%, MOD: 18.84 +/- 1.46%, P< 0.05) or foam cells (ATO: 16.05 +/- 1.26%, HT: 14.88 +/- 1.79%, MT: 16.61 +/- 1.47%, MOD: 22.08 +/- 1.69%, P < 0.05) to the plaque, and an evident increase in content of collagenous fiber (ATO: 16.22 +/- 1.91%, HT: 17.58 +/- 1.33%, MT: 15.71 +/- 2.26%, LT: 14.92 +/- 1.65%, MOD: 9.61 +/- 0.7%, P < 0.05) to the plaque than that in the model group, concomitant with down-regulation of the protein expression of TLR4, MyD88, and NF-kappa B p65, and serum level of MCP-1 and TNF-alpha in a dose-dependent manner. There were no differences in serum TC, LDL, HDL, or TG levels between ApoE(-/-) mice and those treated with atorvastatin. Conclusions: These results suggest that Tan IIA could stabilize vulnerable AS plaque in ApoE(-/-) mice, and this anti-inflammatory and immune-regulating effect may be achieved via the TLR4/MyD88/NF-kappa B signaling pathway.