Boron carbide is one of the hardest materials known. The research on the synthesis of nano-sized boron carbide is of great importance. RF plasma synthesis of B4C nanoparticle was carried out by powder feeder using micro-sized commercial boron carbide powder as the stating material. Different plasma energy and powder feeder probe position was used to find out the optimum condition to achieve nanoparticles. Powders were collected from Chamber 1, 2 and 3. fine particles can be achieved, but the decomposition and oxidation of B4C powder during the high temperature plasma spray was detected. Boron oxide was found in the sprayed powder, and the powder was purified by washing and heat treatment. XRD, SEM and TEM was used to for characterization of the particles.