Effects of RE element La on microstructure and properties of ZL303 alloy have been investigated by means of optical microscope（OM） and mechanical measurements and orthogonal testing. The distribution of RE element La and its compound phase in the ZL 3030 alloy have been observed by SEM （scanning electron microscope）. The results reveal that microstructure of ZL303 can be evidently refined with La addition

in which black skeleton Mg2Si phase is converted into short-rod ones

and gray net-like AlFeMnSi is converted into skeleton ones. Orthogonal testing show that the tensile strength of ZL303 alloy with 1% La addition can be remarkably improved. Compared with that of ZL303 alloy

the tensile strength and elongation of Al-5Mg-Si-La alloy can be increased by 17% and by 120%

respectively

and the hardness can be slightly improved. Majority of La and its compound phase are distributed into the phase interface and grain boundary with minority of them into α-Al phase. In addition

the mechanism of La refining structure and improving mechanical properties of the ZL303 alloy was described.