Proliferation of vascular smooth muscle cells plays an important role in the pathogenesis of atherosclerosis and restenosis. The purpose of this study is to investigate the inhibitory mechanism of trichostatin A(TSA) on the neointimal proliferation in atherosclerosis. To study the anti-proliferative potency of HDAC inhibitor in vascular smooth muscle cells, we utilized the well-established rat carotid injury animal model. TSA inhibited neointima formation 14days after injury in carotid arteries. The intima area and stenosis in the TSA-treated group were significantly reduced compared to the control group. To determine the effect of TSA on SMC proliferation in vitro, we performed the MTT assay and 3[H] thymidine incorporation analysis. TSA significantly decreased PDGF-BB stimulated SMC proliferation in vitro. KLF4 was upregulated by TSA treatment in vascular smooth muscle cells as well as in carotid injury model treated with TSA. KLF4 physically binds to the HDAC4 or HDAC5 and it affected the expression of cell cycle regulators. TSA increased the activity of -1594 KLF4 promoter. These results suggest that KLF4, a novel anti-atherosclerosis regulator, mediates the HDAC inhibitor-induced prevention of vascular smooth muscle cell proliferation.