Most studies conducted on friction stir welded (FSW) Al alloys are on plates that are 2.5-7 mm thick. However, the U.S. Army utilizes materials that are 25 mm thick and greater for structure and armor. In order to properly apply FSW to Al-Cu-Mg-Ag alloys for use in next generation ground vehicles, data must be generated and available for model and simulation databases. One key type of data is the tensile-creep behavior of FSW AA 2139-T8. Creep is the time dependent, plastic deformation of a material under a constant load, usually observed at a constant temperature where T>0.3Tm. The objective of this study is to provide information regarding the tensile-creep behavior of the stir zone in comparison to the heat affected zone (HAZ) through the depth of the weld. The results from this research provide information on the effect of FSW processing on the microstructure and creep behavior. Pre- and post-deformation samples were analyzed via SEM and TEM and the results are discussed.