ZrB 2 -based ceramics, reinforced with 25 vol% SiC whiskers (SiC w ) as well as 0, 2.5, 5 and 7.5 wt% carbon nanoparticles (C np ), were prepared by spark plasma sintering (SPS) at 1900 ºC under 40 MPa for 7 min in a vacuum environment. The influences of C np content on densification behavior, microstructure evolution , hardness and fracture toughness of ZrB 2 –SiC w ceramics were investigated. Compared to the carbon-free sample, the grain growth of ZrB 2 matrix was moderately decreased (~ 20%) after the addition of C np . The in-situ formation of B 4 C and ZrC phases was attributed to the elimination of surface oxide impurities through their chemical reactions with the C np additive. All composite samples approached their theoretical densities. A hardness of 21.9 GPa was obtained for ZrB 2 –SiC w sample, but the hardness values linearly decreased by the addition of soft carbon additives and reached 14.6 GPa for the composite doped with 7.5 wt% C np . The fracture toughness showed another trend and increased from 4.7 MPa m ½ for the carbon-free sample to 7.1 MPa m ½ for 5 wt% C np -reinforced composite. The formation of new carbides and the presence of unreacted C np resulted in toughness improvement. Various toughening mechanisms such as crack branching, bridging, and deflection were detected and discussed.