Among all alloying elements the rare earth elements (RE) play a key role in improving the ductility, high temperature strength and corrosion resistance effectively for magnesium and its alloys. The present work investigated the influences of single alloying or multi heavy REs (Gd, Dy and Y) alloying on the strengthening and ductilizing of magnesium. These heavy REs have a higher solid solubility in magnesium than that the light REs such as Nd and Ce. It is found that the solid solution strengthening caused by Gd follows the linear relationship with the exponent n value of 1/2 or 2/3. When adding two or more heavy REs in Mg, owing to their interactions and resultant synergetic effects, the effectiveness of strengthening and ductilizing caused by multiple RE addition is much better than that by single RE addition.