The microstructure of an Al-4Cu-Mg alloy during isothermal heat treatment in the Strain Induced Melt Activation (SIMA)process was investigated and the kinetics of grain growth was analyzed. The grain growth during isothermal heat treatment of the Al4Cu-Mg alloy coincided with the Ostwald ripening theory. During isothermal heat treatment, both grain shape and the high volume fraction of solid phase have significant effects on grain growth. Therefore, a new grain growth model based on the Ostwald ripening theory was proposed taking into consideration the grain shape and the volume fraction of solid phase. By comparing the calculated results with the experimental results, it was confirmed that the present model could be applied to grain growth during isothermal heat treatment of the Al-4Cu-Mg alloy in the SIMA process.