In situ development of barium hexaferrite, BaFe₁₂O₁₉, particles in 3-mol%-yttria-doped zirconia (3Y-TZP) matrix was studied for the pressureless reactive sintering of 3Y-TZP, BaCO3, and y-Fe₂O₃ powders at 1250°-1400 °C for 2h. Fine-grained BaFe₁₂O₁₉ particles were homogeneously distributed in fine TZP matrix. Changing sintering temperatures could easily control the size of in-situ-developed BaFe12O19 particles. Submicrometer-sized BaFe₁₂O₁₉ particles were obtained at 1250°-1300 °C, allowing BaFe₁₂O₁₉ particles to be single magnetic domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite sintered at 1300 °C showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. The composite also indicated moderately good mechanical properties.