The effects of Sb and Sr on the modification and refinement of Mg_(17)Al_(12)and Mg_2Si phases in Mg-12Al-0.7Si alloy were investigated and compared. The microstructure and mechanical properties of Mg-12Al-0.7Si alloy and its modification mechanism by Sb and Sr were investigated using a scanning electron microscope(SEM), an energy dispersive spectrometer(EDS), X-ray diffraction(XRD) and differential thermal analysis(DTA). The results indicate that by adding 0.5wt.% Sb to the Mg-12Al-0.7Si alloy, the Mg_(17)Al_(12)phase was refined and broken into some discontinuous island structures. However, some network Mg_(17)Al_(12)phases still can be detected in Mg-12Al-0.7Si-0.09 Sr alloy. Therefore, Sb performs better in modification and refinement of Mg_(17)Al_(12)phase than does Sr. Small amounts of fine polygonal shaped Mg_2Si phases were found in Mg-12Al-0.7Si-0.5Sb alloy, while the morphology of Mg_2Si phases in Mg-12Al-0.7Si-0.09 Sr alloy changed from the coarse Chinese script shapes to fine granule and irregular polygonal shapes, indicating that the effects of modification and refinement on Mg_2Si phase are more significant by adding 0.09 wt.% Sr than 0.5wt.% Sb. The ultimate tensile strengths of the Sb and Sr modified Mg-12Al-0.7Si alloys were considerably increased both at room temperature and at 200 oC. The effects of Sb and Sr on the modification and refinement of Mg_ (17) Al_ (12) and Mg_2Si phases in Mg-12Al-0.7Si alloys were investigated and compared. The microstructure and mechanical properties of Mg-12Al-0.7Si alloy and its modification mechanism by Sb and Sr were investigated using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA) % Sb to the Mg-12Al-0.7Si alloy, the Mg (17) Al 12 phase was refined and broken into some discontinuous island structures. However, some network Mg_ (17) Al_ (12) Mg-12Al-0.7Si-0.09Sr alloy. Therefore, Sb performs better in modification and refinement of Mg_ (17) Al_ (12) phase than does Sr. Small amounts of fine polygonal shaped Mg_2Si phases were found in Mg-12Al-0.7 Si-0.5Sb alloy, while the morphology of Mg_2Si phases in Mg-12Al-0.7Si-0.09 Sr alloy changed from the coarse Chinese script shapes to fi ne granule and irregular polygonal shapes, indicating that the effects of modification and refinement on Mg_2Si phase are more significant by adding 0.09 wt.% Sr than 0.5 wt.% Sb. The ultimate tensile strengths of the Sb and Sr modified Mg-12Al-0.7 Si alloys were both greatly increased at room temperature and at 200 oC.