This work presents the results of the Friction Stir Welding (FSW) for two dissimilar thermoplastic polymers: Polypropylene (PP) and ultra-high molecular weight polyethylene (UHMWPE), under butt welding position. The preliminary results show poor welding, consequently, bad mechanical properties; nevertheless, introducing a self-designed and constructed fixed aluminum shoulder and determination of the best FSW parameters, such as advance speed, rotation speed, preheating or no preheating, leads to an improvement on the welding results. Tensile tests for the welded dissimilar specimens PP-UHMWPE, have revealed as high as 80% of welding efficiency in regard the original base materials. Furthermore, measures of microhardness along the welded zone and around were performed; together with the analysis of fracture behavior under tensile tests, in order to deduce the generalities of effect-cause in the improvement of mechanical properties, which are related to the glassy transition temperature of both thermoplastics, the molecular diffusion in liquid state and the process parameters.