Metals of low melting points are usually added to Al-Mg alloys in order to increase the machinability. These metals form eutectic compounds together with Mg, crystallized at grain boundaries. Low melting point mother alloys with three different shapes-wire, ribbon, and powder-having various particle sizes and surface areas were made by varying cooling rates and used to make Al-Mg alloys. The smaller the particle size of the powder was, the higher the size distribution of eutectic compounds was. Ribbons with medium size particles had the value of size distribution between the ones of wire and powder. It is thought that using wire-shaped mother alloys results in statisfactory machinability of Al-Mg alloys.