Stents　have　been　used　successfully　for　treating　stenosis　in　the　vertebral　artery　ostium.　The　size　of　stent　is　found　to　be　an　important　link　in　stent　design,　implantation　strategy,　and　clinical　outcome.　However,　there　is　no　direct　evidence　of　a　relationship　between　stent　expansion　ratio　and　the　stented　artery.　This　study　investigated　the　influence　of　stent　expansion　ratio　on　local　hemodynamics　(such　as　pressure　distribution　and　pressure　gradient)　of　vertebral　artery　ostial　stenosis　to　determine　a　possible　biomechanical　mechanism.　Computer-aided　design　of　models　with　stents　with　different　expansion　ratios　(i.e.,　1.00,　1.125,　and　1.25)　and　internal　flow　fields　were　created.　All　the　models　were　meshed　and　simulated　using　computational　fluid　dynamics　(CFD)　tools.　The　comparisons　of　pressure　distribution　and　pressure　gradient　are　specifically　presented.　The　results　showed　that　the　pressures　increase　and　the　pressure　gradient　decreases　after　stent　implantation.　The　mean　pressure　at　the　stented　region　rises　significantly　with　the　increase　of　stent　oversize.　The　heterogeneity　of　the　pressure　gradient　was　reduced　at　the　stented　region　in　the　case　with　the　expansion　ratio　of　1.125,　whereas　this　effect　was　not　obvious　in　other　expansion　ratio　cases.　Additionally,　the　combination　of　higher　pressure　and　a　lower　pressure　gradient　in　the　case　with　the　expansion　ratio　of　1.125　was　significantly　observed.　This　study　demonstrated　that　the　proper　size　of　stent,　especially　with　regards　to　the　expansion　ratio,　is　an　important　factor　influencing　the　treatment　of　vertebral　artery　ostial　stenosis.　It　is　the　recognition　of　the　necessity　to　consider　the　relationship　between　expansion　ratio　and　stenosis　in　vertebral　artery　ostium.　These　findings　could　help　to　address　the　optimization　of　hemodynamic　performance　for　stent　implantation.