In this study, monolithic B4C, B4C-CNT, B4C-GNP, and B4C-CNT-GNP composites were prepared by ultrasonic stirrer with the additions of 3.5 vol.% carbon nanotubes (CNTs) and 0-3 vol.% graphene nanoplatelets (GNPs) and produced by spark plasma sintering (SPS) method. Samples are produced at 1600-1650 celcius sintering temperatures under 40 MPa for 5-min holding time. The effects of GNP addition on the densification behavior, microstructure, and mechanical properties of B4C-CNTs composites were investigated. The sintering temperature was reduced from 1650 to 1600 degrees C with the addition of CNTs and GNPs separately. Sintering temperature was also 1600 degrees C for the all B4C-CNT-GNP composites. The relative density value was 96%, 97.2%, and 98.4% for the monolithic B4C, B4C-CNT and B4C-GNP composites, respectively. The highest relative density value was obtained as a 97.9% by B4C-CNT-GNP composites. The hardness value was 30 GPa, 32.1 GPa, and 35.4 GPa for the monolithic B4C, B4C-CNT and B4C-GNP composites, respectively. The highest hardness value was 32.3 GPa for B4C-CNT-GNP composites. There was an increase in fracture toughness of B4C-CNT composites with the addition of the GNP up to 2 vol.%. The highest fracture toughness was obtained as 6.20 MPa.m(1/2) for the sample containing 2 vol.% GNP. Increase in fracture toughness resulted from the pull-out of GNPs, bridging the cracks and changing the direction of the cracks by GNPs.