In this research, synergetic and separate influence of nano-carbon black (C.Bn) and SiC on the microstructure and flexural strength of ZrB2 were investigated. So, ZrB2 and ZrB2- 30vol%-based composites containing 10 and 15 vol% C.Bn as well as ZrB2- 15 vol% SiC were fabricated via spark plasma sintering at 1850 °C for soaking time of 8 min under the applied pressure of 35 MPa. Relative density was measured by Archimedes method. Microstructural evaluation was carried out by applying the field emission electron microscopy (FESEM), and flexural strength was measured by three-point bending test. It was found the relative density improves in the presence of C.Bn and SiC especially in synergetic state so that the full densification was gained in Z30Si10C.Bn and Z30Si15C.Bn composites through their reactions with impurities at 1850 °C. In the monolithic ZrB2 system, the C.Bn addition improves the flexural strength slightly to 300 MPa and 315 MPa from 290 MPa. However, co-doped 10 vol% C.Bn with 30 vol% SiC resulted to achieve maximum flexural strength of 486 MPa in comparison with individually applying each of them (395 MPa for Z30Si and 300 MPa for Z10 C.Bn).