In situ(α-Al_2O_3+ZrB_2)/Al composites with network distribution were fabricated using low-energy ball milling and reaction hot pressing. Differential thermal analysis(DTA) was used to study the reaction mechanisms in the Al–Zr O2–B system. X-ray diffraction(XRD) and scanning electron microscopy(SEM) in conjunction with energy-dispersive X-ray spectroscopy(EDX) were used to investigate the composite phases, morphology, and microstructure of the composites. The effect of matrix network size on the microstructure and mechanical properties was investigated. The results show that the optimum sintering parameters to complete reactions in the Al–Zr O2–B system are 850°C and 60 min. In situ-synthesized α-Al2O3 and Zr B2 particles are dispersed uniformly around Al particles, forming a network microstructure; the diameters of the α-Al2O3 and Zr B2 particles are approximately 1–3 μm. When the size of Al powder increases from 60–110 μm to 150–300 μm, the overall surface contact between Al powders and reactants decreases, thereby increasing the local volume fraction of reinforcements from 12% to 21%. This increase of the local volume leads to a significant increase in microhardness of the in situ(α-Al2O3–Zr B2)/Al composites from Hv 163 to Hv 251. In situ (α-Al_2O_3 + ZrB_2) / Al composites with network distribution were fabricated using low-energy ball milling and reaction hot pressing. Differential thermal analysis (DTA) was used to study the reaction mechanisms in the Al-Zr O2-B system X-ray diffraction (XRD) and scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDX) were used to investigate the composite phases, morphology, and microstructure of the composites. The effect of matrix network size on the microstructure and mechanical properties was investigated. The results show that the optimum sintering parameters to complete reactions in the Al-Zr O2-B system are 850 ° C and 60 min. In situ-synthesized α-Al2O3 and Zr B2 particles are uniformly around Al particles, forming a network microstructure; the diameters of the α-Al2O3 and Zr B2 particles are approximately 1-3 μm. When the size of Al powder increases from 60-110 μm to 150-300 μm, the overall surface conta ct between Al powders and reactants decreases, thereby increasing the local volume fraction of reinforcements from 12% to 21%. This increase of the local volume leads to a significant increase in microhardness of the in situ (α-Al2O3-Zr B2) / Al composites from Hv 163 to Hv 251.