High-quality bulk boron carbide (B4C) is successfully prepared at high pressure and high temperature (HPHT) by using B4C powder as a precursor. The as-synthesized B4C possesses a nanosheet structure with a thickness value of 15 nm and a length of several dozen micrometers. Its Vickers hardness value and fracture toughness value are 42.4 GPa and 4.51 MPa·m1/2, respectively, which are superior to those of B4C obtained from spark plasma sintering due to its high densification and nanosheet structure. Additionally, it shows good property of oxidation resistance. In air, its oxidation resistance temperature is 1100?C which is higher than that of diamond under the same test condition.