Zirconium diboride (ZrB2) ceramics were densified by pressureless sintering (PS), hot pressing, or spark plasma sintering (SPS) of powders with a range of starting particle sizes and oxygen contents. Microstructural analysis of the ZrB2 ceramics revealed a wide range of final grain sizes. SPS resulted in an average grain size as small as 1.6 μm after densification at 1900°C, while the largest grains, 31 μm, were produced by PS at 2100°C. Oxygen impurities in boride ceramics caused grain coarsening in all densification techniques, but inhibited full densification only for PS. Carbon was added to react with and remove oxygen impurities, which promoted densification, reduced ZrB2 grain size, and led to increased room-temperature flexure strengths. The highest strength was 527 MPa for SPS ZrB2, while the lowest strength was measured for pressurelessly sintered ZrB2, 300 MPa. Overall, SPS was the superior technique for providing the highest strength and greatest ability to remove oxygen.