The B4C/BN nanocomposites were fabricated by hot-pressing sintering of the B4C/BN nanocomposite powders at 1850oC for 1h under the pressure of 30MPa. The composite powders with the microstructure of B4C particles coated with nano-sized BN particles were prepared by the chemical reaction of H3BO3 and CO(NH2)2 on the surface of B4C particles at high temperature. The microstructure investigation of the nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed in the B4C matrix. With the increasing content of h-BN, the density of the B4C/BN nanocomposites decreased gradually; the fracture strength and fracture toughness of the B4C/BN nanocomposites decreased gradually, the strength and toughness of the B4C/BN nanocomposites with the h-BN content of 10wt% and 20wt% achieved high values. The Vickers hardness of the B4C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the machinability of the B4C/BN nanocomposites was significantly improved. The B4C/BN nanocomposites with the h-BN content more than 20wt% exhibited excellent machinability.