A study was made of the sintering of niobium carbide in electric furnaces. It is shown that niobium carbide parts can be sintered at relatively low temperatures (2400–2500°C) if activating additions are used. An addition of ammonium chloride activates sintering by creating a defective structure, which results in a considerable intensification of the diffusion and bulk viscous flow processes. The experimental data are used for calculating the coefficients of diffusion of niobium in niobium carbide with and without an ammonium chloride addition. The activation energies of the diffusion and sintering processes are calculated. The similarity of these energies is an indication that a diffusion-viscous flow mechanism is responsible for the sintering of refractory carbides.