Finite　element　method　(FEM)　was　used　to　simulate　the　process　of　a　direct　current　(DC)　flash　butt　welding　(FBW)　processing.　In　the　simulation　thermal-electrical　coupling　and　flash　process　are　considered.　Element　live-death　method　was　adopted　to　simulate　metal　melting　and　metal　loss　during　the　flash　process.　Then,　the　temperature　field　of　the　welded　joint　was　computed.　In　addition,　a　Monte　Carlo　(MC)　simulation　technology　was　utilized　to　investigate　the　Austenite　grain　growth　in　the　heat　affected　zone　(HAZ)　of　the　FBW　for　the　ultra-fine　grain　steel.　On　the　basis　of　the　result　of　the　temperature　field,　an　experimental　data-based　(EDB)　model　proposed　by　Gao　was　used　to　establish　the　relation　between　the　MC　simulation　time　and　real　time　in　the　grain　growth　kinetics　simulation.　Moreover,　thermal　pinning　was　considered　due　to　the　effect　of　temperature　gradient　on　HAZ　grain　growth　of　welded　joint.　The　simulations　give　out　the　grain　size　distribution　in　HAZ　of　the　FBW　welded　joint　for　the　ultra-fine　grain　steel　and　effect　of　grain　growth　due　to　temperature　gradient.　(c)　2004　Elsevier　B.V.　All　rights　reserved.