Mg-Al-Sn-Si system alloy, as a promising cheap heat-resistant Mg alloy for automobile engine part, has been investigated. Refinement of microstructure and precipitation of thermally stable secondary phases are important goal for the design of heat-resis-tant Mg alloy. In this study, the effect of Ce on the microstructure and room temperature mechanical properties of Mg-Al-Sn-Si alloy was investigated. High thermally stable Mg2Si phases in Mg-Al-Sn-Si alloy is very useful intermetallic compound. However, the Mg2Si phases often result in poor mechanical properties due to the coarse chinese type Mg2Si phases. The experimental specimens were fabricated by fluxless melting under CO2+SF6 atmosphere and poured into the permanent pre-heated at 200℃. It was told that Ce addition can modify Mg2Si phases and refine microstructure and improve the tensile strength, yield strength and elongation.