In the present work, we present the microstructural study of a cast magnesium alloy AM60B processed by Friction Stir Processing (FSP) in order to achieve Ultra-Fine Grain (UFG) size (200–700nm) in the stirred zone. Focused ion beam (FIB) has been used for sample preparation, and transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED) for the structural characterization. To explain the differences induced by the FSP, two lamellas obtained by FIB from the bottom and the upper part of the cross-section of the nugget are compared to one extracted from the parent material (unaffected zone A). The obtained results show the remarkable homogenization and grain refinement produced in the structure in the processed zone, as previous reported works indicate. The microstructure of the as cast base material reveals the presence of dendrites of solid solution of Al in Mg (α-phase), with an average grain size of ~400–500 µm, and a partially divorced eutectic structure at the interdendritic spaces, where the massive and lamellar β-Mg17Al12 intermetallic phase can be observed (Figure 1). The SAED on one particle of β-phase shows a diffraction pattern that fits the crystalline structure of the cubic I -4 3 m space group (Nº 217). Also Mn containing phases were homogeneously distributed throughout the section. The performed analyses demonstrate that these particles are basically Al-Mn binary phases containing trace elements as Si or Fe, and the measured Mn/Al ratio allow to classify them into two types previously reported: Type I are particles of equiaxed or almost rounded shape and type II are needle like or flowerlike particles, with a lower Mn/Al ratio.