The purpose of this study is to reveal the cause of hardening of magnesium base alloys by the high pressure torsion processing (HPT) using X-ray diffraction. HPT was applied to Mg base alloys of the Mg-Sm system (2.8-5.5 mass %Sm). HPT was performed under pressure 4GPa at 20 oC and 200 oC. HPT results in significant strengthening of the Mg-Sm alloys due to the formation of sub microcrystalline structure. The dynamic recrystallization was realized through the pole figure measurement and the photograph of X-ray back scattering. The Mg supersaturated solid solution decomposition during HPT was observed by the X-ray profile analysis and the calculation of lattice constants of the Mg phase.