Hafnium boride (HfB

2

) 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 HfB

2

ceramic powders were prepared by crystal seed-mediated bor

o/carbothermal reduction method using hafnium oxide (HfO

2

)

boron trioxide (B

2

O

3

)

and carbon (C) powders as raw materials. The influence of crystal seed size on the particle size of HfB

2

powders was studied

and the growth mechanism of HfB

2

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 HfB

2

powders by crystal seed-mediated boro/carbothermal reduction is approximately 1500 ℃ (holding for 1 h). HfB

2

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 HfB

2

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 HfB

2

particle size. It is confirmed that the growth process of HfB

2

ceramic powders is divided into two stages: the cladding of HfO

2

grains on HfB

2

surface with formation of tiny HfB

2

grains

and conversion of HfO

2

to HfB

2

through mass diffusion of carbon and boron from the outside to the inside of HfO

2

particles.