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Boron carbide and its composites find a wide range of applications such as armour material, p-type semiconductor in electronic industries, as a neutron detectors and absorbers in nuclear industry and as a thermo-electric device for space applications, due to their unique physical, thermal and thermo-electric properties. This work discusses about the development of B4C-HfB2 ceramic-ceramic composites. Nearly full dense B4C-HfB2 ceramic composites were fabricated by in situ processing using B4C, HfO2 and CNT, as starting materials. The effect of HfO2 and CNT content on microstructure and mechanical properties of B4C composite has been investigated. Additions of 2.5–30 wt% HfO2 and 0.2–2.5 wt% CNT resulted in improvement in density and fracture toughness of the material. On increasing the additive contents, the fracture toughness of the composite increased more than twice that of monolithic B4C, whereas hardness decreased by about 12 %. Elastic Modulus of the composites was measured to be in the range of 570–625 GPa. Crack deflection observed in the composites was found to be the major toughening mechanism due to the existence of residual thermal stress. The maximum value of hardness, fracture toughness and elastic modulus were 36 GPa, 6.6 MPa m1/2, and 625 GPa, respectively.