In this study, Friction Stir Welding of Ti-6Al-4V was developed in 25 mm thickness material.  The microstructure and mechanical properties, such as static, fatigue, fracture toughness and crack growth, of these thick section Friction Stir Welds were evaluated and compared to Electron Beam Welds produced in the same thickness material.  It was found that the solid state Friction Stir Welds produced a coarse grained transformed beta microstructure with large alpha platelet colonies and grain boundary alpha as a result of super transus processing temperatures and slow cooling rates.  The rapidly solidified structure of the Electron Beam Weld nugget resulted in relatively coarse prior beta grains, but with a much finer acicular alpha structure within the prior beta grain boundaries.  The Electron Beam welds had comparable strengths and fracture behavior compared to the base material, but fatigue properties were degraded due to porosity.  The Friction Stir Welds possessed lower strengths and hardnesses, but improved ductility and comparable fatigue and fracture behavior relative to the base material.  It is intended that this initial data will allow design engineers to down select these processes, considering both cost and performance, for the fabrication of advanced aircraft structures.