Abstract: Aluminum matrix composites reinforced by ZrB₂ particles were fabricated with Al-KBF₄-K₂ZrF₆ components via a melt direct reaction method. The suitable synthesizing temperature and reaction time are 850 to 870℃ and 25 to 30 rain, respectively. Scanning electron microscopy observations indicate that the size of the particles is 300 to 400 nm and the interval between them is about 200 nm. Some of the particles stick together, leading to clusters with a size of 30 to 40 μm. When the theoretical volume fraction of the particles is 3%, the nucleation amount is 6.68 × 10¹⁷ m^-3, and the average linear growth velocity is 47.3 nm·s^-1. The reason for clustering is that abundant fine and infusible ZrB₂ particles increase the viscosity of the composite melt. High diffusion resistance prohibits the particles from migrating. Thus the particles have higher density and sedimentation velocity, which make them cluster easily. In situ reaction analysis shows that the particles can be obtained by molecular combination between Al₃Zr and AlB₂ or atomic combination between Zr and B atoms, and they are stable at high temperature.