The influences of Ce, La, Nd, Y, Sb, Zn and Mn on microstructures of Mg3Al alloys were investigated. The formation of needle-like Al-RE compounds in Mg3Al alloys added 1%RE have close relations with the type of RE element. La and Ce help the acicular phase formation in Mg3Al alloy, which mainly are Al4RE, while, granular Al2RE phases were observed in the Mg3Al alloys with 1%Nd and 1%Y. However, spherical CeSb instead of needle-like Al4RE were found in Mg3Al alloy with 1%Ce and 1%Sb addition. There were not essential differences between the microstructure of Mg3Al-1%Ce-1%Sb and AZ31-1%Ce-1%Sb alloys.