Magnesium alloys have received considerable attention because of their lightweight and recyclability. AM-type and AZ-type Mg-Al alloys have been used for industrial products widely, particularly for AM-type alloys because of the better toughness and impact absorption properties than AZ-type alloys. However, there is little report about the effect of casting method on age-hardening behavior and microstructure of AM-type alloys. The purpose of this study is to investigate the difference of the age-hardening behavior and microstructures of three AM-type alloys cast with steel, copper and sand molds using hardness test and scanning electron microscopy (SEM) observation. Furthermore, the effect of Al content is also investigated in this study using three alloys of AM30 (3%Al), AM60 (6%Al) and AM90 (9%Al).