Aim To explore the role of serum S-adenosylmethionine (SAM)/S-adenosylhomo cysteine (SAH) in vascular atherosclerosis B1 repetitive sequence DNA methylation induced by Hcy in ApoE―/― mice. Methods 36 male Apo E―/― mice fed for 4 months were randomly divided into 3 groups:model control group, high methionine group and folic acid＋VitB₁₂ intervention group, 12 mice in each group, and they were respectively fed with different diets. 12 healthy 4-week-old male C57BL/6J mice were taken as normal control group which were fed with normal diet for 15 weeks.The atheromatous plaque were detected by HE staining, the levels of SAM and SAH in the serum were detected by high performance liquid chromatography (HPLC) and the methylation levels of B1 repetitive sequences by methylmion specific PCR (MSP). Results Compared with the normal control group, serum Hcy levels of high methionine group were significantly increased by 2.39 fold (P＜0.01). The arterio-plaque formed in the high methionine group by HE staining. The expression of SAM/SAH increased by 1.68 and 2.75 fold in the model control group and the high methionine group (P＜0.01). Compared with the normal control group, the methylation levels of B1 repetitive sequences significantly decreased 11.8% in the high methionine group (P＜0.05), and had negative correlation with SAM/SAH(r＝－0.3638,P＝0.0210).Conclusion B1 repetitive sequences in ApoE―/― mice had DNA low methylation and it was negatively correlated with SAM/SAH, which suggested that serum SAM/SAH could be used as a biological index to determine vascular As.