Superplasticity and crystallographic orientation distribution of an Al-Mg-Mn sheet subjected to friction stir processing (FSP) have been investigated. The FSP was developed by adapting the concepts of friction stir welding to obtain a fine grain size in a stirred zone. Grain refinement was achieved by the FSP to give a smaller grain size of about 4µm than a value of 26µm in the as-received sheet. The SEM/EBSP analysis of the specimen before and after the FSP revealed a fraction of high angle grain boundaries became a higher percentage of 83% due to the FSP than 75% before. The elongation to failure in the FSPed sample had a maximum value of 361% at 773K and 1.1×10-3 s-1. Further, change in crystallographic orientation distribution is investigated during tensile testing in consideration of deformation mechanism.