Opting for desirable modifiers is an important way to obtain coast Al-Si-Mg alloys with both high strength and good toughness properties.Structures,mechanical properties and hydroden contents of as-cast and as-heattreated A357 alloy are investigated by comparing modification effections of lanthanum(La)-rich mischmetal and Strontium (Sr).It is shown from our experiments that modification effections of La-rich mischmetal is better than that of Sr as Fe content in the alloy is lower than 0.11% in the aspects of removal of hydrogen from the alloy,refinement of entectic silicons and improvement of mechanical properties,therefore,La-rich mischmetal is a desirable modifier for Al-Si-Mg alloy.