Nano-hafnium diboride powder was synthesized by boro/carborthermal reduction process of HfO2 using a spark plasma sintering (SPS) apparatus. The agglomerated product was found to be 12 mu m in size, which was composed with small primary particles of 100-200nm in size. The particle size of HfO2 played an important role to produce the nano-boride powder. In addition, the grain growth of HfB2 could be effectively suppressed by using SPS due to the fast heating rate. The synthesized powder had a low oxygen content (0.66 wt%). Excessive B4C and C were necessary during the boro/carborthermal reduction in HfO2 process because of the volatilization of boron as a form of B2O3 and the reduction in residual HfO2.