A cryogenic plasticity phenomenon appears in the extruded-T5 treated Mg-9Gd-0.6Zr alloy with tensioning at -196℃. Elongation of the alloy at -196℃ is one time higher than that at 25℃. The lattice constant （c/a values） of Mg-9Gd-4Y-0.6Zr alloy was measured by XRD （X-ray diffraction）. The results show that c/a values of extruded-T5 treated alloys are decreased to 1.6148 with adding Gd and Y elements,and it is further decreased to 1.6113 at -196℃,which results in the occurrence of multi-system slip by decreasing the critical shearing stress of prismatic slip systems at cryogenic tension. So the cryogenic plasticity of the alloy is superior to room plasticity.