Hyperhomocysteinemia, which is an independent risk factor for atherosclerosis, may cause aberrant methylation and dysregulation of gene expression, but the characteristics of the aberrant methylation and its key links involved in its pathogenic mechanisms are still poorly understood. The effect of hyperhomocysteine on DNA methylation in vascular smooth muscle cells, its characteristics and the underlying mechanism of Hcy-induced changing in DNA methylation patterns were investigated. Clinical relevant concentrations of homocysteine was added into the cultured vascular smooth muscle cells of the Homo sapien umbilical vein for 24 h. The level of SAM and SAH was detected by HPLC. The activity of SAH Hydrolase was detected by real-time quantitative reverse transcription-PCR and Western blotting analysis. The level and patterns of DNA methylation were measured by endogenous C-5 DNA methyltransferase(C-5 MT-ase) activity and capacity of genomic DNA to accept methy1 groups and methylation-dependent restriction analysis. The results indicated that an increased Hcy concentration induced elevated SAH, declined SAM and the ratio of SAM/SAH, reduced expression of SAH Hydrolase, but increased activity of C-5MT-ase. The methylation status of gDNA analyzed by methyl-accepting capacity of gDNA uncovered a demethylation process in gDNA, or homocysteine-caused hypomethylation in gDNA. With different methylation-dependent restriction endonucleases, the aberrant demethylation was found to prefer C^↓CGG sequences to CpG islands. The impacts of different dosage of Hcy showed that the varied detrimental effects of Hcy could be attributed to different concentrations via different mechanisms. In mild and moderate hyperhomocysteinemia, the Hcy may primarily influence the epigenetic regulation of gene expression through the interference of transferring methyl-group metabolism, while in more higher Hcy concentration, the notorious impacts may be more directly caused via oxidative stress, apoptosis, inflammation etc.