A master alloy for modification and grain refining of hypoeutectic and eutectic Al--Si based foundry alloys is described. In addition to unavoidable contaminants the alloy contains nucleating and modifying additions of Ti, B and Sr, wherein the content of Ti is between 0.5 and 2.0% by weight, the content of B is between 0.5 and 2.0% by weight and the content of Sr is between 3.0 and 12.0% by weight, with the ratio Ti/B between 0.8 and 1.4. A method for the preparation of said master alloy is also described.