Background and Objectives 竊� PDE5 inhibitor has been shown to inhibit smooth muscle cell proliferation and reduce neointima hyperplasia. Udenafil, one of PDE5 inhibitor, is also expected to inhibit smooth muscle cells proliferation and neointima hyperplasia. We investigated the effect of udenafil on smooth muscle cells proliferation and reduce neointima hyperplasia after balloon injury in the rat carotid injury. Materials and Methods 竊� Smooth muscle cell was treated by udenafil with 100nM, 1uM, 10uM and 100uM. The effect of udenafil inhibit smooth muscle cell evaluated by CCK and MTT assay. Rat carotid arteries were injured with a balloon. Udenafil (10uM and 100uM) applied carotid artery adventitia after balloon injury. At 21 days after treatment udenafil, carotid artery was harvested and stained H & E and elastin. The intima/media (I/M) ratios were measured with image analysis program. Results 竊� In vitro experiment, udenafil treatment reduced Smooth muscle cell proliferation 73% with 100uM. There were no inhibition effect of 100nM, 1uM and 10uM. In balloon injured rat carotid artery, a udenafil group with 100uM reduced the I/M ratio 51% compare control group. Conclusion : In this study, we show that udenafil inhibits smooth muscle cell proliferation and reduces neointima hyperplasia after balloon injury to the rat carotid artery. Keywords: udenafil, carotid artery, neointima hyperplasia, smooth muscle cell proliferation.