ZrB2-based ceramics, reinforced with 25 vol% SiC whiskers (SiCw) as well as 0, 2.5, 5 and 7.5 wt% carbon nanoparticles (C-np), were prepared by spark plasma sintering (SPS) at 1900 degrees C 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 ZrB2-SiCw ceramics were investigated. Compared to the carbon-free sample, the grain growth of ZrB2 matrix was moderately decreased (similar to 20%) after the addition of C-np. The in-situ formation of B4C 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 ZrB2-SiCw 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(1/2) for the carbon-free sample to 7.1 MPa m(1/2) 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.