Titanium nitride and tantalum nitride powders were synthesized by nitriding metal powders under high nitrogen pressures of 1 to 10MPa. Starting metal powders were diluted with corresponding nitride powders to control the reaction temperature and prevent agglomeration of molten particles. The characteristics of combustion reaction in this process were investigated by measuring the propagating velocity of the combustion front for packed reactants with various dilution levels. The nitriding process in combustion reaction was explained as follows; in the combustion synthesis of TiN, nitride layer was formed on molten particles in the early stage of combustion reaction, which prevented coalescing of molten particles. On the other hand, in TaN the combustion proceeded by solid state reaction between metal particles and nitrogen gas. In both cases diffusion of nitrogen atom through the nitride was considered to be a rate-determining step in the combustion reaction.