Grain growth in the semi-solid treatment of an Al-4Cu-Mg alloy has been investigated, which show that kinetics of grain growth in Strain Induced Melt Activated (SIMA) process obeys the Ostwald ripening theory. The grain growth via Ostwald ripening can be analyzed in terms of the classical LSW theory, which was developed by Lifshitz, Slyozov and Wagner. However, the classical LSW theory is only valid for the infinitely separated spherical grains with a low volume fraction of solid phase. In fact, both the grain shape and the volume fraction of solid phase have significant effects on the kinetics of grain growth. In this paper, a new kinetic model to represent the grain growth of the Al-4Cu-Mg alloy in semi-solid treatment was proposed by introducing the volume fraction of solid phase and shape factor of grains. Moreover, a comparison of the experimental data and the calculated shows that the present kinetic model for grain growth could be used in the semi-solid treatment of Al-4Cu-Mg alloy.