Fig. 3 shows the SEM images of the synthesized powders. It can be seen that both ZrB2 powders derived from ZC and ZG have a rod-like morphology. Most of the ZrB2 particles have an aspect ratio of about 3–4. The oxygen contents of the ZrB2 powders synthesized with carbon black and graphite were approximately the same, about 0.8 wt.%. The particle size (Equivalent to diameter of a sphere with the same weight) of the obtained ZrB2 powders was 1.5 and 1 μm when using graphite and carbon black as carbon source, respectively. The difference in particle size can be attributed to two factors: the particle size of the starting carbon source powders and the reaction synthesis temperature. Firstly, carbon black has a much smaller particle size (D50 = 42 nm) than the graphite has (D50 = 1.5 μm). Secondly, the synthesis temperature for the carbon black mixture (ZC) was lower. To explore which reason is essential for the finer particle size, ZrB2 powder was also synthesized at 1500 °C using carbon black. As shown in Fig. 4, the average particle size of the ZrB2 powder additional at 1500 °C was 920 nm, indicating that the powder had almost the same particle size with the one synthesized at 1300 °C for ZC group. Therefore, when using carbon black as carbon source, ZrB2 grains growth is quite slow with increasing temperature. Compared to the grain size of ZrB2 derived from graphite at 1500 C, it can be concluded that the difference in particle size was mainly caused by the raw particle size of the carbon resource.