The structural instability of yttrium hexaboride (YB6) at high temperature limits its application in the ultra-high temperature field. By introducing Yb element, high-temperature stable (Y1-xYbx)B6 solid solution can be formed. In this paper, (Y0.5Yb0.5)B6 powders were prepared via boron/carbon thermal reduction method using (Y0.5Yb0.5)2O3 and B4C as raw materials, and the ceramics were densified by pressureless sintering. The crystal structure, microstructure and mechanical properties were investigated based on the experimental results and density functional theory calculations. The results show that the (Y0.5Yb0.5)B6 powders synthesized at 1 650 ℃ have the highest purity when B4C is excess by 6.25%. (Y0.5Yb0.5)B6 ceramics have a relative density of 95.80% after pressureless sintered at 2 000 ℃, but the grains are coarse with the average size of (80.71±35.51) μm. The relative density, grain size, hardness and fracture toughness of ceramics obtained by two-step sintering are 95.47%, (14.54±6.31) μm, (14.53±1.37) GPa and (2.81±0.34) MPa·m1/2, respectively. The fracture morphology of ceramics is very similar to that of typical high damage tolerance ceramics Ti3SiC2 and Hf3AlN, which indicates that (Y0.5Yb0.5)B6 has good damage tolerance and is expected to improve the toughness and ductility of ultra-high temperature ceramics.