Aluminium-silicon alloys have low density, high electrical and thermal conductivity and high resistance to corrosion at ambient temperature. However, these alloys usually contain numerous alloying and impurity elements, which consist essentially of a fairly ductile matrix of alpha aluminium solid solution with a variety of non-ductile particles of silicon and various intermetallic compounds. The shape and distribution of these constituents largely control the deformation behaviour of the alloy. The addition of magnesium makes the alloys lighter and harder, but its hardening effect is fully responsive only after proper heat treatment. Therefore, in the present study, microstructures and properties of the alloys have been evaluated on the as-cast and heattreated conditions. Results show that the addition of magnesium to aluminium-silicon eutectic base alloy refines microstructure up to certain level and increases the strength and hardness at the expense of ductility.