Aim: Medicinal leech has been widely used as a traditional Chinese medicine in cardiovascular diseases. However, its pharmaceutical effect is not fully revealed. The goal of this study was to determine whether a leech extract has the effect of anti-atherosclerosis in ApoE -/- mice and the mechanism of this effect. Methods and results: In vivo experiments: ApoE -/- mice fed on high-cholesterol diet were separated into 5 groups. Control group was administrated with normal water; leech extract of low dose treatment group was given a leech extract of 0.02 g/kg/d; leech extract of medium dose treatment group was given a leech extract of 0.1 g/kg/d; leech extract of high dose treatment group was given a leech extract of 0.5 g/kg/d; simvastatin group was given simvastatin of 10 mg/kg/d. Leech extract significantly reduced atherosclerotic lesions in aortic root compared with control group. And the number of macrophage in or around the atherosclerosis plaque is significantly reduced in the leech extract groups compared with control group. In vitro experiments: human endothelial cell line, EA.hy926, was induced with TNF-alpha to perform endothelial dysfunction. Control group: EA.hy926 cells with no special treatment; TNF-alpha group: EA.hy926 cells were induced by 10 ng/ml TNF-alpha for 6 h; leech extract only group: EA.hy926 cells were treated with 200 mg/ml leech extract only; leech extract and TNF-alpha group: 200 mg/ml leech extract was applied before TNF-alpha induction. Protein and mRNA level were detected in each group, leech extract can decrease the expression of intercellular adhesion factor (ICAM-1) and monocyte chemotactic protein (MCP-1) compared with TNF-alpha group. Furthermore, it showed less adhesion and migration of THP-1 cells to EA.hy926 cells in the adhesion assay and transwell assay. The NF-kappa B translation to nucleus was blocked by leech extract in the NF-kappa B translocation assay. Conclusions: Leech extract could obviously attenuate the area of atherosclerosis lesion in ApoE -/- mice. And this effect is dose dependent. The effect is mainly a result of reduced invasion of monocyte in artery walls by blocking NF-kappa B translocation. (C) 2014 Elsevier Inc. All rights reserved.