Based　on　the　Pennes　bioheat　transfer　equation,　location　and　flow　rate　of　a　large　blood　vessel　effect　on　microwave　thermal　ablation　was　analyzed,　in　which　the　convection　heat　transfer　of　the　cooling　water　was　involved.　Numerical　results　showed　when　the　distance　between　the　antenna　and　a　blood　vessel　exceeded　20mm;　the　ablation　area　was　elliptical-like.　The　blood　vessel　effect　on　the　ablation　can　be　ignored.　Doctor　needn＇t　operate　the　blood　vessel　blockade　before　microwave　thermal　therapy.　But　when　distance　was　less　than　10mm,　the　other　side　of　blood　vessel　also　has　been　coagulated.　Flow　rate　effect　on　the　ablation　area　is　small.