Hafnium boride (HfB₂) ultra-high temperature ceramics have become one of the best candidate materials in the field of thermal protection due to their high melting point, high oxidation resistance and excellent corrosion resistance. The fabrication of ceramic powders with precisely controlled particle sizes is paramount for their effective application. Highpurity HfB₂ ceramic powders were prepared by crystal seed-mediated boro/carbothermal reduction method using hafnium oxide (HfO₂), boron trioxide (B₂O₃), and carbon (C) powders as raw materials. The influence of crystal seed size on the particle size of HfB₂ powders was studied, and the growth mechanism of HfB₂ powders synthesized by crystal seed-mediated method was explored. Phase and morphology of the obtained powders were analyzed and observed by X-ray diffraction and scanning electron microscope; the contents of C and O impurities in the powders were measured as well. The results show that the optimal reaction temperature for synthesizing pure HfB₂ powders by crystal seed-mediated boro/carbothermal reduction is approximately 1500 ℃ (holding for 1 h). HfB₂ ceramic powders with average particle size of 1.08–2.33 μm were obtained by controlling the initial crystal seed size. Generally, the particle size of HfB₂ increases with the increase of crystal seed size, and laser particle size measurements indicate that the addition of crystal seeds improves the dispersibility and narrows distribution of HfB₂ particle size. It is confirmed that the growth process of HfB₂ ceramic powders is divided into two stages: the cladding of HfO₂ grains on HfB₂ surface with formation of tiny HfB₂ grains, and conversion of HfO₂ to HfB₂ through mass diffusion of carbon and boron from the outside to the inside of HfO₂ particles.