Friction Stir Spot Welding (FSSW) is a variant of the Friction Stir Welding (FSW) process and has been successfully used in industrial applications. During the FSSW process, thermal inputs due to friction and deformation are commenced simultaneously, as the non-consumable rotating tool plunges into the workpiece to be welded. Various assumptions and hypotheses for mechanisms of heat generation and material deformation during FSW/FSSW process are reported, but a consensus is still to be reached. The joining quality is mainly dependent upon the material flow in this solid state joining technique. The material flow and deformations in the near and far fields of the weld are directly affected by the temperature-sensitive mechanical properties. Therefore, a comprehension of thermo-mechanical responses are of high importance from the viewpoints of parameter optimization and understanding of the mechanisms. The FSSW process is experimentally and theoretically studied to address these issues of the mechanism of heat generation and coupled thermo-mechanical response of the workpiece, as well as the effects of tool rotation and plunge speeds. For theoretical studies, a 3-dimensional, physical-based FEM (Finite Element Method) model is developed using commercial code. For heat generation, friction and deformation-based formulations are used. For material responses, thermal and strain rate-sensitive, elastic-plastic data are employed by a constitutive Johnson Cook material model and thermo-mechanical behaviour is analyzed with respect to experimental observations. To cope with high calculation time and distortion of the mesh, built-in features of the code, mass scaling, ALE (Arbitrary Lagrangian Eulerian) and mesh re-mapping were used. As a result of this work, a basic platform in the form of a physical-based, coupled, thermo-mechanical model is developed. With the help of this model, effects of process parameters on the temperature — displacement behaviour of the workpiece are studied. The role of interaction conditions at the tool-workpiece interface is emphasized and a simplified conceptual mechanism for effects of process variables on the physical phenomena is presented.