The commercial software DEFORM-3D was used for the FEM simulation, and Lagrangian-code was employed for the analyses. The workpiece was set to a continuum model of 70 × 70 × 4 mm with meshing into about 32 thousand tetrahedral elements. The element sizes around the tool/workpiece interfaces were set to be about 0.1 - 0.3 mm in length, finer than those in the surroundings. The tool in this model had the same geometries as that used in experiments except that the probe height was 3.6 mm. That is, the probe height was 0.2 mm smaller in the FEM simulation than in experiments and mesh breaking at model’s bottom while re-meshing was avoided by an increase in the clearance between the tool tip and workpiece bottom. The probe part of the model was a cylinder pin without threads, giving priority to analysis stability during FSW simulations. There were 15 thousand tool elements, and tool/workpiece interface elements were set to be finer like the workpiece modeling. The workpiece model had a rigid plastic body and tool model a rigid body, and thermo-coupled viscous-plastic analyses were conducted.