The influence of mischmetal (lanthanum and cerium) on the microstructure and mechanical properties of Mg-9Li-3Al-xRE (x=0, 0.5, 1, 1.5 and 2 wt.%) alloys was investigated. In as-cast alloys, Al4RE phase was formed, the content of Mg17Al12 phase and the volume fraction of α-Mg phase were decreased. In addition, α-Mg phase was refined and the mechanical properties of Mg-9Li-3Al-xRE alloys were improved. However, with increasing mischmetal content, the size of Al4RE phase increases and the mechanical properties decrease. In extruded alloys, the Al4RE phase was broken into the particles of ~1-3μm in the β-Li matrix phase and α/β interphase. And the best mechanical properties was obtained for Mg-9Li-3Al-1.5RE alloy with a tensile strength of 228.3MPa and elongation of 20.8%, increased by 88.6% and 197.4%, respectively, compared with as-cast Mg-9Li-3Al alloy.