Mg-9 wt% Al semi-solid alloys with and without mischmetal were prepared by heating isothermally the billets cast through a cooling pipe, and their microstructures were compared to the case of conventionally cast billets. More spherical and finer solid particles were observed in the alloys with RE, and further refinement could be obtained by using the billet cast with a cooling pipe. The ultimate tensile strength of thixoformed specimen was remarkably increased by the RE addition at 150 degreesC probably because the quantity of Mg17Al12 phase, which is unstable at the elevated temperature, was reduced by the formation of more stable Al11RE3 phase.