Graphene nanoplatelets were incorporated into (20-x)TiC-80ZrC(vol%) composite powder with x = 0, 0.5, 1, 2, 3, 6 and 9 vol% contents. The resulting powder mixture were sintered by spark plasma sintering (SPS) at 1700 degrees C under 40 MPa with 300 s holding time. The prepared composites were then characterized in terms of their densification, microstructure, mechanical properties and phase analysis. By the identification of phases by X-ray analysis, it is verified that Zr4+ ions in the crystal lattice replaced by Ti4+ ions and the substituted solid solution was formed. Almost fully dense ZrC-TiC-GNP composites containing 3 vol% GNP was obtained with a relative density of more than 99%. Vickers microhardness decreased with increasing GNP content, and ZrC-TiC-GNP containing 9 vol% GNP had the lowest value as 13 GPa. On the other hand, fracture toughness increased with increasing GNP content up to 3 vol%, and the maximum fracture toughness value was 5.17 MPa m(1/2).