In this work, electron backscatter diffraction (EBSD) and digital image correlation were employed to study tensile behavior of friction-stir welded magnesium alloy AZ31. The character of measured strain patterns indicated the prevalence of basal slip but also suggested significant activity of prism slip. A substantial strain gradient was found lengthways in the stir zone. The failure was shown to be initiated at the weld root and to originate from double twinning. Crack propagation was influenced by the “onion-ring structure” of the stir zone.