Abstract: An approach of selecting alloying elements to control the size of in situ formed reinforcement in metal melts is proposed. This approach is based on the effect of alloying addition on nucleation of reinforcement predicted by the extended Miedema model and Wilson equation and on growth of reinforcement predicted by diffusion coefficient model. Using this approach, the effect of alloying element addition on the size of in situ formed Mg2Si in (AlN+Mg2Si)/Mg composites is evaluated. The results show that the addition of Ti, V, Mo, Cr, Fe and Mn can delay the nucleation and promote the growth of Mg2Si, while the addition of Ge, Sn, Cu and Zn can promote the nucleation and hinder the growth of Mg2Si. The microstructure of (AlN+Mg2Si)/Mg with the addition of Ti has been investigated based on the theoretical prediction. [Copyright &y& Elsevier]