The in-flight compositional modification of boron carbide pou der with a chemical composition of B5,25C and an average grain size of 22.3 mu m was carried out in an Ar-H-2 RF induction plasma. Reactor pressure, powder feed rate and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. The powder was heated to he partially melted and evaporated during the plasma treatment. The carbon content in plasma-treated powders was decreased to 16.2-16.8 wt.% from that of original one, 17.0 wt.%. The formation of free boron, which agreed with the thermodynamic prediction, and the decrease of cell volume of non-stoichiometric boron carbide BxC were shown in plasma-treated powders. The very fine particles coagulated from a vapor phase consisted of carbon and boron carbide.