Boron carbide (B₄C) based composites with graphene nano-platelet (GNP) as reinforcement have been prepared by hot pressing. Effect of GNP addition on mechanical properties of the composite, like, hardness, fracture toughness, flexural strength was evaluated. Phase evolution was studied by X-ray diffraction (XRD) and Raman spectroscopy. 2 vol% addition of GNP improved the density of the composite, which acted as a sintering aid. Further addition of GNP resulted in formation of the interconnected networks, agglomerates and interfacial porosities. Improved fracture toughness (K_IC) but decreased flexural strength was observed with addition of GNP. Maximum fracture toughness of 5.41 ± 0.55 MPa m^1/2 (by indentation method) and 4.52 ± 0.15 MP am^1/2 (by single edge notch bend (SENB) method) was obtained for 10 vol% GNP-B4C composite. Hardness and elastic modulus were also improved with 2 vol% addition of GNP. The observations have been explained with suitable mechanism and model, supported by microstructural and mechanical property data.