In this work severe plastic deformation (SPD) was applied to magnesium base alloys of the Mg-Sm system (2.8-5.5 mass %Sm). These alloys are characterized by high strength at elevated temperatures and high strengthening effect during aging. SPD was performed by torsion under pressure of 4 GPa at 20 and 200 degrees C to epsilon similar to 6. SPD results in significant strengthening of the Mg-Sm alloys due to the formation of submicrocrystalline structure. In all cases SPD accelerates the solid solution decomposition upon subsequent aging. The highest strengthening can be obtained if the solution treated alloy is aged at 200 degrees C after SPD at room temperature. The state of high strength can be also reached if the following sequence of the operations is used: solution treatment + aging at 200 degrees C up to maximum hardness + SPD at 20 degrees C + aging at 200 degrees C accompanied by Sm -rich phase precipitation in the submicrocrystalline matrix.