With a purpose of increasing the mechanical strength of Al–Si based casting alloys, a ball milling treatment was applied to the rapidly solidified flakes and machined chips. The ball milling was carried out by using an attritor ball mill with addition of methanol as the process control agent. The P/M materials were produced from the ball milled powders by hot extrusion and their structures and mechanical properties were compared with those prepared without ball milling. Attained increases in tensile strength by the ball milling treatment of the rapidly solidified flakes are about 400 MPa for the hypoeutectic and eutectic Al–Si based alloys and less than 250 MPa for the hypereutectic alloy. The strength increases by ball milling are generally much larger than those obtained by rapid solidification. The highest tensile strength of 776 MPa is observed for the rapidly solidified and subsequently ball milled material of the JIS AC8A (ASTM 336.0) alloy. Strengthening attained by ball milling is explained by highly stabilized work hardened substructures due to fine dispersoids formed during ball milling.