The influence of different contents of RE on microstructure of Mg-5Al-1Si alloys was studied. Also, the extracting phases were identified and the extracting process and its influence on the mechanical property of alloy were investigated. The results show that addition of small amount of RE to Mg-5Al-1Si alloys resulted in refinement of microstructure. The morphology of the Mg2Si phase changed form coarse Chinese script shape to the fine polygonal shape. Owing to the improvement of microstructure, the mechanical properties of Mg-5Al-1Si alloys at both ambient and elevated temperatures were increased . The high temperature creep resistant properties of Mg-5Al-1Si-xRE raises with the increase of RE content and higher than that of AE42.