Advanced welding methods like Friction Stir Welding (FSW) continue to emerge along with methods for reducing the structures weight. Since the design of the structures is becoming complex, the need for welding of complicated geometries is increasing. However, finite element modelling of FSW is still limited to the flat welding seams. On the other hand, since significant thermal energy and high stress is generated during FSW, thermomechanical analysis of FSW plays a significant role in determining the quality of the weld. Thus, there is a need investigate the thermomechanical analysis of FSW with a curved welding path. Thermal behavior of the flat and the curved FSW is compared in this paper. The results indicated that, the temperature and stress behavior is approximately asymmetric in different sides of the tool. Moreover, as the welding seam becomes more complex, the size and the shape of the different welding zones significantly changes. It is also observed that at a same condition, the temperature and the stress of the curved model is always lower in comparison with the flat model due to the lower penetration of the pin inside the workpiece. Finally, verification and validation of the results is done by using the experimental and published data. © 2019, CIMNE, Barcelona, Spain.