Ultra-high temperature ceramic composites of ZrB2-SiC were densified by pressureless sintering. Ultra-fine ZrB2-SiC composite powders synthesized by sol-gel method were mixed with commercial ZrB2 and SiC powders. The sintered body of hybrid powders (combined commercial and synthesized composite powders) showed excellent properties not only in the relative density but also the flexure strength. Dry-pressed compacts using 4wt% Mo as a sintering aid were sintered to nearly full density at 2200°C/2h. The average strength was ~560MPa and the maximum was ~632MPa. SEM and TEM showed that SiC particles were distributed homogenously in the ZrB2 matrix and the average particle size was ~5μm. From HRTEM observations, the grain boundaries were apparently free of glassy phases and no intermediate phases existed.