Abstract: Atherosclerosis(AS), characterized with the accumulation of lipids on the vessel wall, is an immune-related inflammatory disease which promotes the progression of cardiovascular diseases(CVD). The imbalance of Treg/Th17 accelerates the progression of AS. Yangyin Huoxue Prescription(YHF)is an efficient traditional Chinese medicine used in the treatment of AS, but the effects of YHF on the balance of immunity have still not been clarified. This project was designed to investigate the effects of YHF on the imbalance of Treg/Th17 and AS in ApoE^-/- mice induced by high-fat diet(HFD). ApoE^-/- mice were given HFD to induce AS and administered low-dose YHF(18 g/kg)or high-dose YHF(36 g/kg)for 20 weeks. Atherosclerotic plaque area was analyzed by oil red O staining. Serum lipids were measured by biochemical kits. Treg or Th17 cells in peripheral blood were detected by flow cytometry. mRNA and protein expression of Foxp3 and RORγt of aortas were determined by qRT-PCR, Western blot and immunohistochemistry. Splenic CD4⁺T cells of mice were isolated and activated by anti-CD3/CD28, and then treated with lipopolysaccharide(LPS)and YHF. The expression of mRNA and protein of Foxp3 and RORγt were detected by qRT-PCR and immunofluorescence. It was found that YHF reduced the plaque area, decreased lipid level and increased the ratio of Treg cells in peripheral blood. Moreover, YHF increased mRNA or protein expression of Foxp3 in aortas in vivo or CD4⁺T cells in vitro while decreasing mRNA or protein expression of RORγt. These results suggested that YHF can regulate the imbalance of Treg/Th17 in ApoE^-/- mice induced by HFD, and reduce the inflammatory stimulation of LPS on CD4⁺T cells, thereby improving AS.