The microstructure of ZrB2/ZrC/Zr composites was examined using scanning electron microscopy, optical microscopy, and X-ray diffraction techniques. Dense ZrB2/ZrC/Zr composites could be fabricated by the reaction sintering of molten zirconium with ZrB2 preform. The composites were made by infiltration of molten zirconium into ZrB2 preform, which contained 0-40 vol % B4C, at 1900 degrees C for 10 min. The average grain size of ZrB2 in the reaction-sintered composites decreased slightly with an increase in the volume fraction of the B4C. The volume fraction of the solid increased with further increase of B4C contents. The mechanical properties were measured in accordance with B4C contents. The composites exhibited a four-point bending strength of up to 570 MPa and a fracture toughness of up to 11.5 MPam(1/2).