The microstructural and mechanical characterization of two alloys within the Mg-Ni-YRE system prepared by casting and subsequent hot extrusion at 400°C have been carried out. The microstructure of both materials consists of a fine-grained magnesium matrix embedding a high volume fraction of second phases; coarse Mg12RE and long period ordered stacking structure (LPS phase) and fine Mg2Ni particles. Both alloys show high strength values up to 250°C. The yield stress values at room temperature are 295 and 405 MPa for low- and high-alloyed magnesium alloy, respectively. Load transfer from the magnesium matrix to coarse Mg12RE and LPS particles account for the high strength of both alloys at temperatures below 250°C. Above this temperature both alloys exhibit a superplastic behaviour at low stresses with elongations of 700 and 450 % for the low and high-alloyed magnesium alloy, respectively.