The　stenosis　at　the　beginning　segment　of　the　vertebral　artery　accounts　for　the　first　risk　of　stroke　in　the　posterior　circulation.　The　extracranial　vertebral　arteries,　especially　the　proximal　ends,　have　been　considered　to　be　the　predilection　sites　of　stenosis　or　occlusion.　From　the　perspective　of　hemodynamics,　the　mechanics　of　vertebral　arteries　stenosis　is　still　unclear.　In　this　paper,　the　formation　of　atherosclerosis　in　proximal　end　was　concerned　from　the　aspects　of　the　effect　of　bifurcation　diameter.　Different　models　represent　different　bifurcation　diameter.　In　order　to　find　correlation　between　bifurcation　diameter　and　WSS　we　build　different　models.　Three　idealized　models　with　the　vertebral　artery　diameter　of　3.6　x　10(-3)　m　(Model　A1),　4.0　x　10(-3)　m　(Model　A2)　and　4.4　x　10(-3)　m　(Model　A3)　respectively　and　seven　realistic　models　were　analyzed　by　using　computational　fluid　dynamics　tools.　The　area　of　low　wall　shear　stress　(WSS,　＜　1.5　Pa)　in　the　proximal　end　of　vertebral　artery　extracted　at　the　peak　systole　in　the　idealized　models　were　2.25　e-7,　8.55　e-7　and　1.61　e-6m(2),　respectively.　The　area　of　low　WSS　on　the　vertebral　artery　origin　of　realistic　models　extracted　at　the　peak　systole　were　0,　1.18　e-09,　3.91　e-07,　1.68　e-07,　5.46　e-06,　1.16　e-06　and　2.25　e-06m(2),　respectively.　Moreover,　the　time-averaged　WSSs　of　the　three　idealized　models　were　3.95,　3.56　and　3.19,　respectively.　The　time-averaged　WSSs　of　the　realistic　models　were　6.28,　6.36,　4.48,　4.71,　3.59,　3.59　and　3.31　Pa,　respectively.　With　the　increase　of　bifurcation　diameter,　the　risk　of　endothelial　dysfunction　increases,　and　the　same　is　to　intimal　hyperplasia.