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2024 aluminum matrix composites reinforced with different size AlN particles (0.5, 4 and 10 μm) were fabricated by the squeeze-casting technology. The aging behavior and microstructure of AlNp/2024Al composites were investigated by Brinell hardness measurement and transmission electron microscopy (TEM). The results show that the precipitation sequence of AlNp/2024Al composites is similar to that of the matrix alloy aged at 160 and 190 ℃, but the age hardening rate of composites is improved, and the AlN particles with large size promote the precipitation process more obviously, in comparison with smaller AlN particles. With increasing temperature, the precipitation processes are accelerated, and the time to reach the peak hardness is shortened. The acceleration of the formation of GP region and phase S in the composites is attributed to the interfaces (between particles and the matrix) and the high density of dislocations introduced by addition of AlN particles.