In　the　present　work　the　evolution　of　grain　structure　in　the　weld　HAZ　(heat　affected　zone)　under　welding　thermal　cycle　was　simulated.　Especially　the　grain　growth　in　the　HAZ　of　a　SS400　ultra　fine　grain　steel　was　investigated.　An　integrated　3-D　Monte　Carlo　(MC)　simulation　system　for　grain　growth　of　the　weld　HAZ　was　developed　based　on　Microsoft　Windows.　The　results　indicate　that　MC　simulation　is　an　effective　way　to　investigate　the　grain　growth　in　weld　HAZ.　The　method　not　only　simulates　the　non-isothermal　dynamics　process　of　the　grain　growth　in　the　weld　HAZ,　but　also　visualizes　the　austenite　grains　realistically.　Moreover,　the　thermal　pinning　effect　can　be　easily　included　in　the　simulation　process.　The　grain　sizes　of　the　CGHAZ　(coarse　grain　heat　affected　zone)　obtained　from　MC　simulation　are　basically　in　agreement　with　the　experimental　measurement　of　the　real　welded　joints　under　different　heat　input.　Furthermore,　the　simulation　indicates　that　the　grain　growth　degree　is　higher　for　the　SS400　ultra　fine　grain　steel　compared　to　conventional　steel.　With　the　increase　in　the　heat　input,　the　grain　growth　of　the　CGHAZ　rapidly　increases.　Because　the　activation　energy　of　the　grain　growth　is　lower　for　the　SS400　ultra　fine　grain　steel,　austenite　grains　can　grow　at　a　relatively　lower　temperature,　hence　the　range　of　the　CGHAZ　becomes　wider.　(C)　2004　Elsevier　B.V.　All　rights　reserved.