In this study, the austenitic grain size in bead-on-plate heat-affected zone (HAZ) are predicted as grain size has been widely known as an important factor affecting the deformation mechanism of materials, its microstructures and mechanical properties. At the first stage, a numerical model of bead-on-plate process using Goldak’s double ellipsoid heat source model is used to assess the temperature distribution during and after welding of austenitic stainless steel SS316L filler wire and plate. The numerical computation is conducted based on temperature-dependant materials properties using commercial FEM software MSC Marc/Mentat with user subroutine. Further, a numerical model is developed by using ordinary differential equation (ODE) for calculating the free grain growth algorithm combined with the presence of growing precipitates. The initial grain size (D0) value was obtained from optical microscopy observation while other values such as modified kinetic constant (M0) and activation energy (Qa) are defined through experimental investigation with various temperature ranges and holding times. It can be concluded that the austenite grain growth prediction algorithm during the bead-on-plate welding thermal cycle was successfully executed. As the outcome, grain sizes were predicted and compared with experimental investigation.