In　the　present　research,　the　Monte　Carlo　technique　was　used　to　simulate　the　grain　in　the　heat-affected　zone　(HAZ)　of　an　ultrafine　grain　steel.　An　experimental　data　based　(EBD)　model　proposed　by　Gao　was　used　to　establish　the　relation　between　tMCS　and　real　time　temperature　kinetics　in　our　simulation.　The　simulations　give　out　the　evolution　of　grain　structure　and　grain　size　distribution　in　the　HAZ　of　the　ultrafine　grain　steel.　A　Microsoft　Window　based　on　computer　program　for　the　simulation　of　grain　growth　in　the　HAZ　of　weldment　in　three　dimensions　has　been　developed　using　the　Monte　Carlo　technique.　For　the　system,　inputting　the　temperature　field　data　and　material　properties,　the　evolution　of　grain　structure,　both　image　of　simulated　grain　structure　and　numerical　data　reflecting　the　grain　size　distribution　can　be　produced　by　the　program.　The　system　was　applied　to　the　ultrafine　grain　steel　welding,　and　the　simulated　results　show　that　the　ultrafine　grain　steel　has　a　large　tendency　of　grain　growth.