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Coarsening kinetics of Pb phase in a nanocomposite alloy produced by mechanical alloying in immiscible Al-Pb system and the influence of Cu addition on it have been studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that when annealing at 573, 623, 673 and 723 K for different times, the relation of average particle size of Pb phase in Al-Pb nanocomposite alloys to time obeys the cube law even though the size of the constituent phase is in the nanometer range. The coarsening activation energy of Pb phase is 84.80 kJ/mol, close to the grain boundary self-diffusion activation energy of Al matrix. This indicates that the coarsening of Pb phase is controlled by grain boundary diffusion. Adding Cu decreases the coarsening rate of Pb phase, primarily by reducing the interfacial energy through Cu segregation at the Al/Pb interface. The coarsening activation energy of Pb phase is increased by the addition of Cu.