The　graft-host　diameter　ratios　have　impacts　on　the　flow　patterns　of　bypass　graft.　In　order　to　clarify　the　influence　of　graft-host　diameter　ratios　on　the　flow　patterns　and　the　wall　shear　stress　in　coronary　artery　bypass　graft　(CABG),　the　pulsatile　blood　flows　in　three　CABG　models,　with　the　graft　diameter　larger　than,　equal　to　and　smaller　than　that　of　the　coronary　artery,　were　simulated　with　finite　element　method.　The　temporal-spatial　distributions　of　flow　patterns,　wall　shear　stresses　(WSS),　wall　shear　stress　gradients　(WSSG),　oscillating　shear　index　and　shear　stress　ratio　were　depicted　and　compared.　Of　the　three　models　evaluated,　large　model　can　bring　about　better　hemodynamics　to　some　extent　with　relatively　large　positive　longitudinal　velocity,　uniform　and　large　WSS,　and　small　WSSG.　The　results　suggest　that　larger　or　isodiametric　graft　is　favorable.　However,　no　distinct　difference　of　WSS　based　temporal　parameters　was　found　between　all　the　three　models.　Alternative　anastomotic　designs　are　necessary　for　the　improvement　of　CABG　patency　rates.