Zirconium diboride (ZrB2) reinforced by nano-SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB2–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB2–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB2 to over 545 MPa for ZrB2–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m1/2 to more than 6.81 MPa·m1/2 in the same composition range. The relative density of ZrB2–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si3N4; however, the effects of different sintering additives on the mechanical properties of the composites were different.